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Grate-firing of biomass for heat and power production

Chungen Yin , Lasse A. Rosendahl, Søren K. Kær

Institute of Energy Technology, Aalborg University, DK-9220 Aalborg East, Denmark

a r t i c l e i n f o

Article history:

Received 20 December 2007

Accepted 9 May 2008

Available online 27 June 2008

Keywords:

Biomass

Grate-fired boiler

Pollutant emission

Particulate matter

Deposit formation

Corrosion

CFD

Fluidized bed

a b s t r a c t

As a renewable and environmentally friendly energy source, biomass (i.e., any organic non-fossil fuel)

and its utilization are gaining an increasingly important role worldwide. Grate-firing is one of the main

competing technologies in biomass combustion for heat and power production, because it can fire awide range of fuels of varying moisture content, and requires less fuel preparation and handling. The

basic objective of this paper is to review the state-of-the-art knowledge on grate-fired boilers burning

biomass: the key elements in the firing system and the development, the important combustion

mechanism, the recent breakthrough in the technology, the most pressing issues, the current research

and development activities, and the critical future problems to be resolved. The grate assembly (the

most characteristic element in grate-fired boilers), the key combustion mechanism in the fuel bed on

the grate, and the advanced secondary air supply (a real breakthrough in this technology) are

highlighted for grate-firing systems. Amongst all the issues or problems associated with grate-fired

boilers burning biomass, primary pollutant formation and control, deposition formation and corrosion,

modelling and computational fluid dynamics (CFD) simulations are discussed in detail. The literature

survey and discussions are primarily pertaining to grate-fired boilers burning biomass, though these

issues are more or less general. Other technologies (e.g., fluidized bed combustion or suspension

combustion) are also mentioned or discussed, to some extent, mainly for comparison and to better

illustrate the special characteristics of grate-firing of biomass. Based on these, some critical problems,

which may not be sufficiently resolved by the existing efforts and have to be addressed by future

research and development, are outlined.& 2008 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 726

2. Biomass as a fuel for grate-fired boilers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 727

3. Grate-firing: a suitable technology for biomass combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728

3.1. Fuel-feeding system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729

3.2. Grate assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 729

3.3. Primary air and traditional combustion mechanism in the fuel bed on the grate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 730

3.4. Advanced secondary air supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 731

3.5. A comparison between grate-firing and FBC for biomass combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7324. Key issues associated with grate-firing biomass: R&D in progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733

4.1. Primary pollutant formation and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733

4.1.1. Pollutants from incomplete combustion and the control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733

4.1.2. NO x emissions and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734

4.1.3. HCl and SO x emissions and control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 735

4.1.4. PCDD/PCDF emissions and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736

4.1.5. Particulate matter and heavy metals emissions and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 736

4.2. Deposit formation and corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

4.2.1. Deposition indices based on fuel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

4.2.2. Mechanisms of deposit formation and high temperature corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

ARTICLE IN PRESS

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journal homepage: www.elsevier.com/locate/pecs

Progress in Energy and Combustion Science

0360-1285/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.pecs.2008.05.002

Corresponding author. Tel.: +459940 9279; fax: +45 98151411.

E-mail address: [email protected] (C. Yin).

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4.2.3. Possible solutions to the problems of deposition and high-temperature corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 740

4.3. Modelling and CFD simulations for diagnosis, optimization, and new design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742

4.3.1. Modelling of biomass conversion in the fuel bed on the grate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742

4.3.2. CFD modelling of the mixing and combustion in the freeboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 747

4.3.3. Modelling of NO x formation and emissions from grate-fired boilers burning biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748

4.3.4. Modelling of deposit formation in grate-fired boilers burning biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748

4.3.5. Modelling or assessing of the discontinuous effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748

5. Future R&D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 748

5.1. Mechanism study of combustion chemistry for grate-firing of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7485.2. Advanced monitoring, testing, and experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749

5.3. General and comprehensive model for biomass conversion in the fuel bed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749

5.4. Advanced CFD modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 749

5.5. Optimization and modernization for better performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751

1. Introduction

The worldwide concern with global warming, because of the

emission of CO2 and other greenhouse gases and the limited

availability of fossil fuels, has spurred interest in using biomass asa fuel for energy production. The agreements, signed up by the

European Union (EU) in March 2007, to a binding EU-wide target

to source 20% of the energy needs from renewables such as

biomass, hydro, wind, and solar power by 2020 (8.5% in 2007) [1],

will further boost the use of biomass in power production.

Grate firing is one of the main technologies that are currently

used in biomass combustion for heat and power production.

Grate-fired boilers can fire a wide range of fuels of varying

moisture content and show great potential in biomass combustion

[2–4]. Though grate firing of biomass has been tried and tested

over many years, there are still some problems to be further

studied, for instance, conversion of biomass in the fuel bed on the

grate, mixing in both the fuel bed and the freeboard, deposit

formation and corrosion and their control, pollutant formation

and control, modelling and simulations for a better understanding

of the details in grate-fired boilers.

There are some review papers in the literature in which

biomass combustion is covered to different extents. For example, a

comparison was made of the combustion of coal and municipal

solid waste (MSW), in terms of fuel characteristics, combustion

technology, emissions, and ash utilization and disposal [5]. The

combustion of sewage sludge [6] and agricultural residuals [7]

was reviewed, and the important issues of fuel processing,

combustion, and emission characteristics, as well as the handling

of solid by-products, were discussed. Knowledge, mainly on

suspension co-firing coal with biomass, was summarized [8].

The understanding of the combustion of pulverized coal and

biomass from the viewpoint of computer modelling was reviewed

[9]. The chlorine-associated, high-temperature corrosion and thepotential corrosion problems associated with burning biomass

fuels were discussed [10]. The understanding of fuel nitrogen

conversion in solid fuel (mainly coal, but also biomass) fired

systems was reviewed. The effect of parameters that possibly

affect the oxidation selectivity towards NO and N2 was empha-

sized [11]. Combustion characteristics of different biomass fuels,

the potential applications of renewable energy sources as the

prime energy sources in various countries, and the problems

associated with biomass combustion in boiler systems were

discussed [12,13].

The objective of this paper is to provide a state-of-the-art

overview of the grate-firing of biomass for heat and power

production. Grate-fired boilers are often labelled ‘‘high carbon-in-

ash, low efficiency and high emissions’’. Firing of biomass couldalso bring new problems to the combustion systems, e.g.,

deposition and corrosion. The efforts in the area of grate-firing

of biomass may be grouped as follows:

Pollutant emissions: The incomplete combustion gives rise to

higher emissions of CO, hydrocarbons (C xH y), tar, poly aromatichydrocarbons (PAH) and incompletely burned char from

biomass combustion in grate-fired boilers. The relatively high

contents of specific elements (e.g., Cl, S, and heavy metals) in

some biomass fuels may aggravate the pollutant emissions by

emitting HCl, SO x, polychlorinated dibenzo-dioxins (PCDD),

polychlorinated dibenzofurans (PCDF), and heavy metals. Fuel

NO x is the major source of NO x from biomass combustion in

grate-fired boilers. The important NO x precursors (e.g., NH3,

HCN, and NO) released from the fuel bed on the grate, which

are directly related to the atmosphere and the propagation

speed of the ignition front in the fuel bed, play a vital role in

NO x emissions from grate-fired boilers.

Deposit formation and corrosion: Grate-firing of some biomass

fuels with a high Cl content (e.g., straw) may suffer from severe

deposition and corrosion problems. Deposits reduce both the

heat transfer ability of combustor surfaces and the overall

process efficiency, while corrosion reduces the lifetime of the

equipment. Deposition and corrosion depend not only on fuel

properties but also on combustion environments (e.g., atmo-

sphere, temperatures, and mixing).

Modelling and computational fluid dynamics (CFD) simula-

tions, which could represent the majority of the design efforts

devoted to grate-firing of biomass: Basically, the modelling can

be split into two parts: modelling of biomass conversion in the

fuel bed on the grate and modelling of the mixing, combustion,

deposit, and pollutant formation in the freeboard. In the fuel

bed, the propagation of the flame fronts is of practical interest,

as it determines the releases of volatiles, and affects the heat

output from a given grate area and the stability of combustion.The flame propagation also plays an important role in the

release of NO x precursors, particulate matter formation pre-

cursors, and other pollutant formation precursors. So knowl-

edge of the combustion in the fuel bed is important for

optimizing the gas-phase combustion above the bed. The

modelling efforts on the combustion in the freeboard have

mainly focused on how to optimize the mixing in order to

improve burnout and lower the emissions.

Experimental work is another important contributor to the

study of biomass combustion: Experimental results provide

not only valuable insights into the combustion process, but

also the necessary input for modelling, as well as the data for

the validation of models. Comparatively, the experimental

facilities or techniques are more general for any kind of combustors. Moreover, the experimental studies are spread

ARTICLE IN PRESS

C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754726



throughout all different issues or problems. Therefore, the

experimental efforts are not highlighted explicitly as a separate

subject in this review. However, it never means there is a lack

of experimental efforts in the grate-firing of biomass. In fact,

the three issues to be highlighted in this review, i.e., pollutant

emissions, deposition and corrosion, modelling and CFD

simulations, are all related to experimental efforts to different

extents. Pollutant formation and control, particularly therelease of the inorganic elements under grate-firing conditions,

the particulate matter and heavy metals emissions from grate-

fired boilers, and some of the emission control techniques

applicable for grate-fired boilers, are heavily based on experi-

mental studies. Deposit formation and corrosion in grate-fired

boilers, as well as their control, are almost exclusively derived

from experimental results. Some measures, which have been

successfully tested for other combustion technology (e.g.,

fluidized bed combustors) to mitigate deposition and corro-

sion, are not necessarily applicable to grate-fired boilers, due to

the substantially different mixing, stoichiometric conditions

and time–temperature inside the fuel bed. Modelling and CFD

simulations are also closely related to experiments, for the

purpose of validation. Almost all the modelling works onbiomass conversion in the fuel bed on the grate are validated,

to different degrees, by experiments.

This review paper is organized as follows. Firstly, biomass fuels

are briefly introduced: the advantages of firing biomass, the

properties of biomass, the considerations for biomass power

projects, as well as a list of reported examples of biomass fuels

fired in grate boilers. Then, grate-firing technology is presented.

Amongst the key elements of a modern grate-fired boiler, the

grate assembly and advanced secondary air system are high-

lighted. The traditional combustion mechanism in the fuel bed on

the grate is also highlighted, followed by a comparison with

fluidized bed combustion (FBC) of biomass. After that, the three

issues are surveyed and discussed, primarily pertaining to grate-fired boilers burning biomass: (1) primary pollutant formation

and control, (2) deposit formation and corrosion, and (3)

modelling and CFD simulations for the diagnosis and optimization

of existing grate-fired boilers and design of new grate-fired

boilers. Combustion chemistry and physics constitute the founda-

tions of all the issues or problems. Pollutant emissions, as well as

deposit formation and corrosion, are more related to combustion

chemistry, whilst modelling and CFD simulations may be more

related to combustion physics, particularly the mixing (fluid

mechanics), thermodynamics, and heat transfer in the freeboard,

and development of physical models (including sub-grid model).

Both combustion chemistry and physics will be discussed

throughout the different sections rather than highlighted as a

separate issue. Experimentation, more combustion physics-based,

is also discussed throughout the different sections. Finally, a few

critical problems in grate-firing of biomass, which may be

addressed by further research and development (R&D), are

suggested on the basis of the discussions.

2. Biomass as a fuel for grate-fired boilers

In general, any organic non-fossil fuel can be considered a

biomass fuel. Biomass for grate-firing can be mainly grouped into

waste products and dedicated energy crops [8,14]. Waste products

include wood materials (e.g., saw dust, wood chips, wood logs,

and bark), crop residues (e.g., wheat straw, rice straw, corn husks),

and municipal and industrial wastes of plant origin (e.g., MSW,

refuse-derived fuel (RDF), manure). Dedicated energy crops areagricultural crops that are solely grown for use as biomass fuels,

e.g., short-rotation woody crops like hard-wood trees and

herbaceous crops like switchgrass. These crops have very fast

growth rates and can therefore be used as a regular supply of fuel.

Biomass fuels are one of the most important energy resources.

Biomass constitutes 14% of the global primary energy, the fourth

largest following coal, oil, and natural gas. Biomass is the most

important source of energy in developing countries, providing

about 35% of their energy demand [12]. The potential, on a globalbasis, to supply biomass for the production of renewable energy is

assessed and reviewed in [15].

Biomass fuels are also considered environmentally friendly.

Firstly, there is no net increase in CO2 for combustion of biomass if

it is replanted. Biomass consumes the same amount of CO 2 from

the atmosphere during growth as is released during combustion.

Secondly, firing biomass brings additional greenhouse gas mitiga-

tion, by avoiding CH4 release from the otherwise landfilled

biomass. CH4 is 21 times more potent than CO2 in terms of global

warming, based on mass and a 100-year period [8,16]. Thirdly,

most biomass fuels have very little or no sulphur and, therefore,

net SO2 emissions can be reduced if high-sulphur coal is replaced

with low-sulphur biomass. When co-firing with high-sulphur

coals, the alkaline ash from biomass can also capture some of theSO2 produced during combustion [17,18]. Fourthly, some of the

biomass fuels, e.g., wood and paper, typically contain much less

nitrogen on mass basis as compared to coal, and for biomass,

significant amounts of NH3 may be released directly from the

solid matrix during devolatilization [11]. Ammonia helps to

reduce NO to N2, which essentially provides an in-situ thermal

DeNO x source. Lastly, soil and water contamination due to

landfilled or stockpiled biomass can be mitigated by firing

biomass fuels.

The physical and chemical properties of biomass span over a

very broad range. All biomass is composed of three main

components (i.e., cellulose, hemicellulose, and lignin) [9], and a

number of minor components (e.g., lipids, proteins, simple sugar,

starches, water, ash) [19]. The fractions of each class of component

vary depending on species, type of plant tissue, stage of growth,

and growing conditions. The fuel properties (e.g., proximate

analysis, ultimate analysis, ash analysis, and trace elements) of

different biomass have been widely reported or reviewed in the

literature, for example, [7,8,12,18–28]. Biomass contains carbon

(C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S), and

chlorine (Cl), as well as major ash-forming elements (Al, Ca, Fe, K,

Mg, Na, P, Si, Ti) and minor ash-forming elements (As, Ba, Cd, Co,

Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Tl, V, Zn). Among the ash-forming

elements, Ca and Mg usually increase the ash melting point, while

K decreases it significantly. Chlorides and low melting alkali- and

aluminosilicates may also significantly decrease the ash melting

point [27–29]. The fractions of the different elements vary from

one biomass fuel to another. Generally speaking:

(1) the C contents of wood fuels (including bark) are higher than

those of herbaceous biomass;

(2) coniferous and deciduous wood, and paper have the lowest N

contents, while grains and grasses usually contain the highest

N contents;

(3) the Cl content of wood is generally very low, while

significantly higher amounts of Cl are present in herbaceous

biomass, grains, and fruit residues;

(4) straw, cereal, grass, and grain have low contents of Ca and

high contents of K and Si in ash.

The different properties of biomass fuels in comparison with

solid fossil fuels result in different reactivities, combustion,deposition and emission behaviours, as studied and concluded

ARTICLE IN PRESS

C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 727



in, for example, [30–37]. The differences may be summarized as

follows:

Biomass fuels generally have higher volatile contents, less

carbon and more oxygen, and lower specific heating value in

kJ/kg than coal.

Pyrolysis starts at a lower temperature for biomass fuels.

The fractional heat contribution by volatiles in biomass is of the order of 70% compared to 36% for coal.

Biomass fuels, for example, straw, have more free alkali in ash,

which may aggravate the slagging and fouling problems.

Compared to coal chars, biomass chars have higher oxidation

reactivity, probably as a result of the presence of alkalis

(catalytically active) in the char matrix [32].

All these have significant influence on the thermal utilization of

biomass fuels and the choice of the appropriate combustion

technology.

For biomass conversion, a correct evaluation of available

combustion technology options is not sufficient to ensure a

successful biomass power project. A number of other considera-

tions, some of which may be even more crucial, are required toturn biomass into productive heat and/or power, such as

evaluating the availability of suitable biomass resources and

determining the economics of collection, storage, and transporta-

tion. Some biomass fuels may be available for a few weeks per

year, and therefore have to be stored for use throughout the year,

which is clearly different from fossil fuels. Some biomass fuels

may need pre-handling before being fed to combustors for heat

and power production, e.g., leaching, drying, or pelletization.

Moreover, biomass fuels are generally bulky and therefore the

availability of biomass feedstocks in close proximity to a biomass

power project is a critical factor in their efficient utilization. The

primary reasons for the failure of biomass power projects are

changes in fuel supply or demand (wrongly estimated during the

planning stage) and changes in fuel quality [38].

Amongst the numerous biomass fuels given in literature,Table 1 lists some of those which are reported to have been fired

in grate boilers or tested for grate-firing. The original data

collected from the literature, which are not necessarily complete,

are converted into the same basis to ease the comparison, i.e., as-

received basis for proximate analysis and dry-ash-free basis for

ultimate analysis. Not surprisingly, they show great variations in

their chemical properties. Even for the same type of biomass fuels,

their chemical properties may also differ greatly, depending on

the growing conditions (e.g., the place and the season). One can

also observe from the table the diversity of the biomass fuels

currently fired in grate boilers.

3. Grate-firing: a suitable technology for biomass combustion

The two most common types of boilers for biomass combus-

tion are grate-firing systems and fluidized bed combustors, both

of which have good fuel flexibility and can be fuelled entirely by

biomass or co-fired with coal. Suspension burners are often used

to co-fire milled biomass pellets or raw biomass with pulverized

coal or natural gas, in which the air-dried biomass fuels (with

ARTICLE IN PRESS

Table 1

Compositions and heating values for selected biomass fuels fired in grate boilers or tested for grate-firing

Type of biomass fuels (country) Reference Proximate analysis (wt%, as received) Ultimate analysis (wt%, dry and ash-free) Calorific value (MJ/kg)

Moisture VM FC Ash C H O N S Cl Gross CV LCV

Bark (Sweden) [39] 50.00 1.70 51.88 6.10 41.71 0.31 9.00

Bark (the Netherlands) [40] 52.16 6.0 0 41.5 9 0.2 5

Brassia carinata (Spain) [41] 8.88 4.75 48.23 6.32 44.01 1.12 0.32

Coconut shell (India) [42] 6.50 48.15 38.85 6.50 53.21 6.20 39.25 1.28 0.05

Dried sewage sludge (Poland) [43] 3.00 50.00 10.00

Fibreboard (Austria) [44] 10.60 1.48 47.54 6.15 43.19 3.11 15.52

Grape waste (Spain) [45] 11.45 62.51 20.74 5.30 49.87 6.68 40.80 2.50 0.15 16.37

Miscanthus (UK) [46] 6.10 67.90 13.10 12.90 49.26 7.78 42.96 15.40

MSW (UK) [47,48] 36.00 32.00 8 .20 23.80 5 0.20 5.80 42.30 0.97 0.73 7.66

MSW (75%) and RDF (25%) (Germany) [49] 29.00 39.41 4.97 26.63 41.75 5.80 52.45 8.30

Olive husk (Spain) [45] 12.20 64.77 15.87 7.16 50.80 6.05 38.14 4.83 0.18 15.58

Pine (UK) [46] 5.50 81.20 12.10 1.20 53.38 8.68 37.94 18.30

Pine wood (Korea) [50] 25.00 10.00 48.04 7.13 44.84 10.03

RDF (Italy) [51] 20.00 60.77 8.18 11.05 58.28 5.07 33.04 1.42 0.88 1.31 15.00

RDF (UK) [46] 1.90 69.60 9.80 18.70 55.79 7.93 36.27 22.30

Reed canary grass (UK) [52] 8.05 83.87 3.75 4.33 50.45 6.52 42.24 0.80 0.00 16.41Rice (sic) straw (Denmark) [41] 7.40 7.04 47.46 6.36 45.31 0.68 0.18

Straw (the Netherlands) [40] 4 8.52 5.7 0 45.17 0.61

Straw (Poland) [43] 10.70 4.30 15.25

Straw (UK) [52] 7.88 80.08 6.76 5.28 50.18 6.31 42.38 0.69 0.44 16.36

Sugarcane trash (India) [42] 4.00 55.98 38.27 1.75 49.87 5.99 44.13

Switchgrass (UK) [52] 6.43 82.84 7.24 3.49 48.33 6.07 44.55 0.48 0.57 17.26

Waste wood (the Netherlands) [40] 49.90 5.73 42.93 1.45

Wheat straw 2000 (Spain) [41] 7.48 4.70 49.08 6.48 43.60 0.63 0.20

Wheat straw 2005 (Denmark) [53] 12.00 69.52 14.39 4.09 49.24 6.40 43.90 0.46 15.21

Wheat straw (UK) [54] 16.00 63.50 15.00 5.50 49.17 6 .50 42.93 0.76 0.13 0.51 14.58

Willow (UK) [46] 7.20 78.10 13.70 1.00 50.00 7.19 42.81 17.8

Wood (Poland) [43] 35.00 4.00 10.90

Wood (Sweden) [55] 10.00 76.23 13.50 0.27 49.30 6.30 44.40 16.51

Wood chips (Austria) [44] 38.70 0.53 49.07 6.09 44.33 0.50 10.21

Wood chips (Finland) [56] 45.00 46.75 7.15 1.10 50.00 6.12 43.88 10.45

Wood chips (India) [42] 7.00 54.52 38.11 0.37 49.01 6.40 4 4.59

Wood chips (the Netherlands) [40] 50.87 5.9 6 43.05 0.12

Wood chips (Sweden) [57] 30–40 52.00 6.00 41.00 0.60




relatively low moisture content, e.g., o25wt%) must be finely

pulverized (e.g., particle feed size o6 mm) and have a relatively

low share (e.g., less than 25% on energy basis) [3,4,7,38].

Suspension-fired burners are also more sensitive to changes in

fuel quality: firing off-specification fuels can significantly influ-

ence the performance of the boilers [58]. Moreover, suspension-

fired burners require more fuel-handling and preparation equip-

ment, especially when firing pulverized biomass pellets.

Grate-firing is the first combustion system used for solid fuels.

Now it is used mainly for burning biomass, but also for smaller

coal furnaces. Capacities of grate-fired boilers range from 4 to

300MWe (many in the range of 20–50 MWe) in biomass-fired

combined heat and power (CHP) plants. The heat release rate per

grate area may be up to about 4MWth/m2 as a result of high

volatile and low ash characteristics of typical biomass fuels [38].

Fig.1 shows two modern grate-fired boilers, giving an overall view

of the typical arrangement of grate-fired boilers. Basically, modern

grate-fired boilers consist of four key elements: a fuel feeding

system, a grate assembly, a secondary air (including over-fire air

or OFA) system and an ash discharge system.

Here, the key elements in grate-fired boilers are discussed first,

and the grate assembly and the advanced secondary air system are

highlighted. The former represents the most specific component

in grate-fired boilers, whilst the latter is one of the real

breakthroughs in grate-firing technology. The traditional combus-

tion mechanism in the fuel bed on the grate is also highlighted.

This is followed by a comparison between grate-firing and FBC of

biomass, and then a conclusion.

3.1. Fuel-feeding system

Typical fuel-feeding systems used in biomass-fired grate

boilers are mechanical stokers, as shown in Fig. 1. For biomass

fuels that are very heterogeneous in size and contain a relatively

big mass fraction (30% or higher) of fine particles (i.e., a few

millimetres and smaller), a spreader is needed to reduce the

tendency for fuel size segregation since the grate is typically only

suitable for coarse particles. The finer biomass particles combustin suspension when they fall against the upwardly flowing

primary air. The remaining heavier and bigger pieces fall and

burn on the grate surface [60].

3.2. Grate assembly

The grate, which is at the bottom of the combustion chamber

in a grate-fired boiler, has two main functions: lengthwise

transport of the fuel, and distribution of the primary air entering

from beneath the grate. The grate may be air-cooled or water-

cooled. The water-cooled grate requires little air to cool (primary

air confined to combustion requirement) and is flexible with the

use of an advanced secondary air system. The grates are mainly

classified into stationary sloping grates, travelling grates, recipro-

cating grates, and vibrating grates, the major characteristics of

which are summarized in Table 2.

Table 3 lists a few examples of grate-fired boilers equippedwith the different types of grates for biomass combustion, from

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Table 2

Different types of grates and their main characteristics

Type of grate The major features

Stationary sloping

grate

The grate does not move. The fuel burns as it slides down the

slope under gravity. The degree of sloping is an important

characteristic of this kind of boiler. Disadvantages: (1) difficult

control of the combustion process; (2) risk of avalanching of

the fuel.

Travelling grate The fuel is fed on one side of the grate and is burned when the

grate transports it to the ash pit. Compared to stationary

sloping grate, it has improved control and better carbon

burnout efficiency (due to the small layer of fuel on the grate).

Reciprocating

grate

The grate tumbles and transports fuel by reciprocating

(forward or reverse) movements of the grate rods as

combustion proceeds. Finally, the solids are transported to the

ash pit at the end of the grate. Carbon burnout is further

improved due to better mixing.

Vibrating grate The grate has a kind of shaking movement that spreads the fuel

evenly. This type of grate has less moving parts than other

moveable grates (and thus lower maintenance and higher

reliability). Carbon burnout efficiency is also further improved.

Fig. 1. Examples of grate-fired boilers burning biomass. (a) MSW-fired reciprocating-grate boiler [59]. (b) Straw-fired vibrating-grate boiler [53].




which one can also see the capacities of the grate-fired boilers as

well as the biomass fuels fired.

Amongst the different types of grates, vibrating gates may have

the longest life and highest availability. Fig. 2 shows a laboratory-

scale water-cooled vibrating grate [59]. The grate is made of panel

walls with drilled holes in the fins for the primary air. The grate is

part of the boiler pressure system and connected to furnacewalls by flexible connection pipes, designed for the vibrations.

Instead of a continuous ash discharge in a travelling grate, a

vibrating grate utilizes an intermittent ash removal system

where the grate surface vibrates at high frequency and low

amplitudes for about 2% of the time to move the solids forward

and discharge the ash at the end of the grate. As can be seen from

the figure, this type of grate uses very few moving parts and

the drive mechanism is outside the heat and flame, which

increases grate life, reduces maintenance costs and results in

high equipment availability.

The grate assembly can be optimized to significantly improve

the boiler performance, for instance, by allowing a better under-

grate primary air distribution, by enhancing the mixing of

biomass fuels on the grate, and by improving fabric seal of thesystem to lower air leakage. Fig. 3 shows a close view of a modern

grate system, which was built into a brand new grate-fired boiler

burning MSW in 2006 [72].

3.3. Primary air and traditional combustion mechanism in the fuel

bed on the grate

The design of air supply system (primary air and secondary air)

plays a very important role in the efficient and complete

combustion of biomass. For grate-firing, the overall excess air

for most biomass fuels is normally set to 25% or above. The split

ratio of primary air to secondary air tends to be 40/60 in modern

grate-fired boilers burning biomass, instead of 80/20 in older

units, which leaves much more freedom to advanced secondary

air supply.

The primary air distribution, together with the movement of

the grate, affects significantly the mixing and biomass conversion

in the fuel bed. Though some grate-fired boilers burning biomass

may have a low buildup of materials on the bed, the majority of

the biomass-fired grate furnaces in the literature, for example

those as listed in Table 3, may be interpreted as a cross-flowreactor, where biomass is fed in a thick layer perpendicular to the

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Table 3

Grate-firing of biomass: some examples

Type of grate Fuel fired Grate-fired boilera Reference

Stationary sloping grate Wet wood chips 40 MW industrial boiler [61]

Martin-type moving grate MSW 12 T/h MSW incinerator [47,48]

Inclined moving grate Waste 4 T/h industrial boiler [62]

Travelling grate Coal/biomass blends (bagasse, wood chips, sugarcane trash, coconut

shell)

18.68 MWe power plant boiler [42]

Travelling grate Bagasse, wood chips, rice husk 120 T/h, 87 kg/cm2, 515 1C industrial boiler [4]

Travelling grate Wood chip, rice husk, bark, sugar cane trash, cotton stalks, groundnut

shells

35T/h, 66 kg/cm2, 495 1C industrial boiler [4]

Travelling grate Bagasse 90 T/h, 45 kg/cm2, 480 1C industrial boiler [4]

Travelling grate Wood VU-40 industrial boiler [63]

Air-cooled travelling grate Blends of urban waste, natural wood waste, agricultural waste 20MWe industrial boiler [3]

Recipr ocat ing grate Wood chips, b ark , sawdus t, p ellets of s awdust 15 0 r eciprocating grate b oilers (45 MWth each) [64]

Forward reciprocating

grate

MSW 25 MWe utility boiler [65–67]

Vibrating grate Wheat straw 33 MWe utility boiler [54]

Water-cooled vibrating

grate

Wood pellets 500 kW laboratory-scale [68]


grate

Straw 105 MWth boiler [69,70]


grate

20% wood chips and 80% straw 33 MWfuel CHP unit (producing 8.3 MWe and 20.8MJ/s

heat)

[71]

Water-cooled vibratinggrate

Wheat straw 108 MWfuel CHP unit (producing 35 MWe and 50MJ/sheat)

[53]

a A unified description of the boiler capacity would be more helpful. Different descriptions (e.g., main steam parameters, feeding rate of biomass, thermal megawatts or

electrical megawatts) are used here due to the lack of the unified information in the references.

Fig. 2. A laboratory-scale water-cooled vibrating grate [59].




primary air flow. The bottom of the biomass bed is exposed to the

preheated primary air while the top of the bed is within the

furnace. The fuel bed consists of a huge number of solid particles

that are piled up on the grate with a characteristic porosity. The

fuel bed is heated by over-bed radiation from flames and

refractory furnace walls until it ignites on the top surface of thefuel bed. The propagation of the ignition front in the bed is of

practical interest, as it determines the releases of volatiles, and

affects the heat output from a given grate area and the stability of

combustion. It is also directly related to the release of volatile

nitrogen species (NH3 and HCN) and NO formed from volatiles. So

knowledge of the factors influencing the speed of the ignition

front is important for optimizing gas-phase combustion in the

freeboard.

The generally accepted combustion mechanism of cross-

current units may be described as follows [73–76]. After ignition,

a reaction front propagates from the surface of the bed down-

wards to the grate against the direction of the primary air.

The heat, generated in the reaction front, is transported against

the combustion air flow and dries and devolatilizes the raw

fuel. This allows the reaction front to propagate. Due to the

opposing directions of the heat flow and the air flow, the

heat is not transported downwards far from the position

where it is released, and the reaction front is narrow. The heat

generated in the reaction front originates from oxidation

of fuel and, if not all oxygen is consumed in the narrow reaction

front, a char layer will be formed above the reaction front.

When the reaction front reaches the surface of the grate, a

secondary reaction front (i.e., a char burnout front), propagating

upwards to the surface of the bed, burns the char layer previously

formed.

This traditional combustion behaviour in the fuel bed on the

grate may not always be observed. Fuel properties (e.g., moisture,

heating value, and particle size) and operating conditions (e.g.,

primary air flowrate) have significant influence on the combustionbehaviour in the fuel bed [74–78]: the main findings will be

summarized later in the modelling and simulation section. In

some extreme cases, different combustion behaviour may be

observed. For instance, in the combustion process of wet biofuels

(50 wt% moisture content, which was 5–15% beyond the limiting

moisture content corresponding to the primary air flowrate) in a

31 MW reciprocating grate furnace (a cross-current flow combus-

tor), the ignition was found to occur close to the grate, followed by

a reaction front propagating from the grate upwards to the surface

of the bed, which is opposite to the traditional mechanism [75].

Depending on the primary air supply rate, three modes of

combustion in the biomass bed were identified [79]: oxygen-

limited combustion under low air supply rate, reaction-limited

combustion when increasing primary air supply, and extinction byconvection if the primary air flowrate is increased further. In

modern grate-fired boilers burning biomass, the biomass combus-

tion in the fuel bed is more likely under substoichiometric

conditions (i.e., fuel-rich), because biomass fuels have typically

higher volatiles content on a dry basis. Moreover, the multiple

zones of under-grate primary air are often used, which can help to

achieve a more favourable temperature distribution, high ashburnout, and low emissions.

3.4. Advanced secondary air supply

Advanced secondary air supply system is one of the most

important elements in the optimization of the gas combustion in

the freeboard, for complete burnout and lower emissions, e.g., by

forming local recirculation zones or rotating flows and by forming

different local combustion environments (e.g., fuel-rich or oxy-

gen-rich). It is probably the most flexible way to retrofit the old

grate-fired boilers for a better burnout and lower pollutant

emissions [63]. The gases released from biomass conversion on

the grate and a small amount of entrained fuel particles continue

to combust in the freeboard, in which the secondary air supply

plays a significant role in the mixing, burnout, and emissions.

Advanced secondary air-staging is also often used in modern

biomass-fired grate boilers. The basic idea of air-staging is to

reduce NO x formation by reducing oxygen availability in the flame

and by lowering flame temperature peaks. In air-staged combus-

tion process, the first air-deficient (i.e., fuel-rich) zone reduces

NO x formation, and the complete combustion is achieved only

after the addition of OFA in the second zone (i.e., the burnout

zone).

As an example, Fig. 4(a) shows the advanced secondary air

supply in the straw-fired vibrating-grate boiler [53], from which

one can see the enhanced air-staging in the lower furnace. Fig.

4(b) gives a close-up view of the secondary air nozzles on the

front wall in the lower furnace. The nozzles have differentdiameters, spacing, and orientations. The eight secondary air

nozzles on the top level (i.e., OFA), four on the front wall and four

on the rear wall, are staggered. The staggered arrangement of OFA

jets can provide an effective curtain of combustion air, and can

also form a double rotating flow on the horizontal cross-sections

in the burnout zone, as shown in Fig. 4(c), which prolongs the

residence time of the combustibles, distributes the temperature

more evenly, and leads to a better burnout. The characteristics of

the combustion air supply and the combustion zone in such a

grate-fired boiler is sketched in Fig. 5. The majority of the

combustibles are released into the freeboard from the first half

grate and the enhanced air-staging forms a local fuel-rich

combustion environment in the front-bottom part. The air jets,

located on the rear wall in the lower furnace, form a local air-richenvironment and a stable recirculation zone in the rear-bottom

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Fig. 3. A brand new grate-fired boiler burning MSW [72].




corner, both of which help stabilize the combustion on the last

section of the grate and reduce the incompletely burned char in

the bottom ash.

During the advanced air-staged combustion process, the

mixing, temperature, residence time, and local-stoichiometry

play key roles. Besides the optimization of the SA jets (e.g., in

terms of amount, momentum, diameter, location, spacing,

orientation), the static mixing devices with or without air

injections, as shown in Fig. 6(a), may be an effective option to

enhance the mixing [2] in biomass-fired grate boilers. Actually,

the ‘Eco’-tube air system is just such a device with air injection,which generates a considerable improvement in efficiency and a

big NO x reduction in different grate-fired boilers burning biomass

[39,80]. Tangential arrangement of SA jets, as shown in Fig. 6(b),

may also be a good option for grate-fired boilers. The tangentially

arranged air jets form a strong rotating flow on the horizontal

cross-sections, which can not only result in a good burnout but

also mitigate the deposit formation and corrosion on the furnacewalls as a result of the local oxidative conditions and the air

curtain formed close to the furnace walls [81].

3.5. A comparison between grate-firing and FBC for biomass

combustion

FBC is also a competing technology in biomass combustion for

heat and power production. Examples of biomass-fired FBC

projects can be seen in, for example, [82], including both

circulating fluidized bed combustion (CFBC) and bubbling

fluidized bed combustion (BFBC). A general evaluation of BFBC

and CFBC for biomass combustion is that BFBC is good enough

for firing biomass alone; when biomass is co-fired with coal,

CFBC may be needed to guarantee proper char burnout.Table 4 gives a brief comparison of the main characteristics of

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Fig. 4. The advanced secondary-air staging system in a biomass-fired grate boiler [53].

Fig. 5. Sketch of the air supply and the resulted different zones in a grate-fired

boiler burning biomass.

Fig. 6. Different options of advanced secondary air supply in grate-fired boilers. (a)

Static mixing devices with or without air injections. (b) Tangentially arranged SA

jets.




grate-fired boilers and fluidized bed combustors when they are

used for biomass combustion. Some of these points may be found

in [7].

In conclusion, grate-firing systems, particularly modern grate-

fired boilers, are one of the combustion technologies suitable for

biomass combustion for heat and power production. Grate-firing

systems are concluded as a good or even the preferred technology

for biomass-to-energy conversion in many applications, e.g.,

[2,45].

4. Key issues associated with grate-firing biomass: R&D

in progress

R&D activities on biomass combustion are progressing rapidly,

partly because of the similarities with coal combustion and the

many years’ experience and knowledge in coal combustion. For

instance, the mechanisms of pollutant formation, ash formation,

deposition, and corrosion during biomass combustion can all be

traced to some extent to the corresponding mechanistic study in

coal combustion. However, biomass fuels fall over a very broad

range and have quite different chemical and physical properties,

which not only result in different combustion and emission

characteristics but also cause some practical problems during

combustion in different plants. In this section, the three key

issues, primary pollutant emissions and control, deposit forma-

tion and corrosion, and modelling efforts for design and

troubleshooting, are highlighted and discussed, primarily relatedto grate-fired boilers burning biomass.

4.1. Primary pollutant formation and control

Primary pollutants from combustion include NO x, SO x, CO,

C xH y, tar, HCl/Cl2, PAH, PCDD/PCDF, heavy metals, particulate

matter, and incompletely burned char particles [27,91,92]. These

pollutants can be classified into two groups: one from the

incomplete combustion or oxidation and the other from the

inorganic species in the biomass fuel, as listed in Table 5.

4.1.1. Pollutants from incomplete combustion and the control

Incomplete combustion is, most likely, a big problem or

challenge for grate-fired boilers, particularly old units, comparedto FBC or suspension-fired boilers. So the pollutant emissions due

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Table 4

A comparison of grate-firing systems and fluidized bed combustors

Water-coole d mo ving -grate systems Fluidized b ed combustors

Fu el flexibility They can fire heterogeneous fu els wit h large p art icle

sizes, and high moisture contents (up to 65%).

Comparatively, a grate needs more adjustments to fit a

particular fuel than an FBC.

They can combust a wide range of fuels having different sizes,

shapes, moisture (up to 65%) and heating values [3].

Solid mixing and combustionintensity in the fuel bed

Average for travelling grates; good for reciprocating orvibrating grates. Very common combustion instabilities

in the fuel bed [53,64].

Very intense solid (bed materials and solid fuels) mixing, leading tovery uniform temperature distribution and higher combustion

intensity in the fuel bed.

Bed agglomeration Quite many biomass fuels have the low melting point

characteristics (because of the high content of

potassium in ash) [83].

Grate-firing systems are insensitive to fuel bed

agglomeration.

Very sensitive to bed agglomeration, leading to de-fluidization and

unscheduled shut-down [84]. Using silica sand as bed materials, bed

defluidization is experienced when firing coffee husks, sunflower

husks, cotton husks/stalk, mustard stalk, soya husks, pepper waste,

groundnut shell and coconut shell [85], wheat straw [83]. The

problem may be mitigated by special additives or bed materials

[86]. No such problem exists in firing rice husk [87,88], wood chips

[86], wood waste [89], palm fibre [85].

Wear to bed components Little Extensive wear to bed components due to high solid velocity [3].

Emissions They can achieve low NO x emission by using advanced

SA (including OFA) system.

Very low NO x emission from CFBC, mainly due to the char inventory

in the circulating bed materials: char efficiently reduces NO x. In

BFBC, NO x is more difficult to control and requires sophisticated airsystems and also often selective non-catalytic reduction (SNCR).

Very low SO x emission from both CFBC and BFBC due to sulphur

capture by addition of limestone into bed materials.

Fly ash Very low dust load in flue gas; high levels of

incompletely burned carbon in fly ash [13,90].

Solid load in CFBC is at least 100 times higher than in grate-fired

systems because the bed materials follow the flue gases to a large

extent. Solid separation equipment is required. Good burnout of fly

ash. Some toxic materials are bound in fly ash, which is one of the

main challenges in fluidized bed combustion of some biomass fuels,

e.g., MSW.

Partial load operation Good operation is possible at partial loads. Partial load operation requires special technology.

Capital cost Medium to low High for CFBC. BFBC is less expensive than CFBC.

Operation & maintenance

costs

Medium for travelling grates, and very low for vibrating

grates.

CFBC has high operating costs due to higher pressure drop over the

dense bed, and high maintenance costs due to, e.g., the extensive

wear (erosion).

Table 5

Pollutant emissions from biomass combustion

Orig in P ollutant emissio ns Typical bio mass fuels

Group 1— from combustion process

Incomplete

combustion

CO, C xH y, tar, PAH,

unburnt char

All biomass fuels

Oxidization NO x, N2O All biomass fuels

Group 2— from inorganic species in biomass fuel

Ash Particulate matter All biomass fuels

Cl and S HCl, SO x, salts (KCl etc.) Waste wood, straw, grasses, fruit

residues

High Cl f raction PCDD, PCDF MSW, wast e wood, st raw, cer eals

etc.

Heavy metals Pb, Zn, Cd, Cu, Cr, Hg etc. Urban waste wood, sludge




to incomplete combustion tend to be a more prominent topic

associated with grate-fired boilers. These emissions may also

aggravate the formation of other pollutants. For instance, the

incompletely burned char in fly ash, together with the relatively

high excess air in grate-fired boilers, may also lead to high

emissions of PCDD/PCDF when firing some biomass fuels (e.g.,

wastes or wood).

The comparatively poor mixing, both in the fuel bed and in thefreeboard, is the main reason for the incomplete combustion in

grate-fired boilers. Advanced air supply systems and optimized

grate systems can significantly enhance the mixing, reduce the

excess air, improve the combustion process, and lower the

pollutants. For instance, a 1975 vintage travelling grate stoker

boiler in a paper mill in Louisiana, USA, was retrofitted, mainly by

improving air supply system and fabric stoker seal. CO levels were

reduced from over 1000 ppm to 260 ppm after the retrofit as a

result of the improved mixing and combustion. A net reduction of

60% in the pre-retrofit fly ash disposal rates was also observed

[63], which could be due to the reduced char particles elutriated

from the fuel bed, or the reduced unburnt char in the fly ash, or a

combination of the both.

Sufficient residence time of the combustibles in the combus-tion zone is also an important factor for complete combustion in

grate-fired boilers. A 20 MWe air-cooled travelling grate combus-

tion system was commissioned in 2003 in Germany, which

utilizes a combination of urban waste, natural wood waste, and

agricultural waste, with a combined fuel moisture content of

35–40%. Due to the relatively high contaminants contained in the

fuel(s), the regulators directed that considerations of incineration

and reduction of ash disposal take a higher priority, compared to

the gaseous emissions. The design for the boiler, grate, secondary,

and primary air should be capable of meeting the required CO

regulations and, more importantly, should ensure all particles

would obtain 8501C for a minimum of 2s for the design

composite fuel [3]. These requirements are exactly the same as

the EU directive on waste incineration [2,93].

Since grate-firing systems have relatively low combustion

temperatures, good mixing and sufficient residence time of the

combustibles at high temperatures are particularly crucial to

improve the combustion. In water-cooled grate boilers, the grate

requires little primary air to cool. As a result, the flue gas leaves

the fuel bed at a lower superficial velocity and carries away less

combustible particles from the fuel bed. Water-cooled grate

boilers are also flexible with the use of advanced secondary air

system, which can be optimized to enhance the mixing and

improve the combustion in the freeboard and, therefore, lower the

pollutants from incomplete combustion. For instance, a measure-

ment campaign was done on a 108 MWfuel straw-fired water-

cooled vibrating grate boiler built in 1999. The CO measured in the

flue gas at the boiler exit and the boiler efficiency are about

150ppm and 91.7%, respectively, at 100% load. The performance of this boiler could have been further improved to achieve even

lower pollutant emissions due to incomplete combustion and a

higher efficiency [53].

So the pollutant emissions due to incomplete combustion from

grate-fired boilers can be effectively controlled by an optimized

combustion process, i.e., enhanced mixing, sufficient residence

time (at least 41.5 s) at high temperatures (4850 1C), and low

total excess air [27], as well as the appropriate choice of grate

assembly.

4.1.2. NO x emissions and control

NO x emissions from combustion systems can be formed from

different mechanisms [11,94]. As a result of the comparatively lowcombustion temperatures in grate-fired boilers burning biomass,

the yields of thermal NO x can be considered to be small or

negligible, and fuel NO x is the major source of NO x [11,95].

Fuel-N in biomass or coal is released in three stages

[94,96–99], as depicted in Fig. 7. In the first stage, the volatile-N

is released in the primary pyrolysis together with the majority of

volatiles. The major NO x precursors during biomass pyrolysis

include NH3, HCN [100], and HNCO [101,102]. Due to the high O/N

ratio in biomass fuels, part of fuel-N is found to be directlyconverted to NO during primary pyrolysis stage [103]. In the

second stage, the thermal cracking and combustion of volatiles

(mainly tar) provides additional sources of HCN and NH3. In the

third stage, during combustion of the char residue the char-N

mainly forms NO while the rest is converted to N2. The NO formed

may be effectively reduced to N2 over biomass char as a result of

its catalytic effect on NO formation and reduction [104,105].

Therefore, the partitioning of fuel-N between the volatiles and

the remaining char during devolatilization is potentially impor-

tant for final NO x formation. The split between the volatile-N and

char-N is roughly proportional to the volatile matter in the fuel

[7]. Because biomass fuels have higher yields of both light gases

and tar, and lower char yields, a comparatively larger fraction of

the fuel-N may be released with the volatiles. Latest experiments,for a broad range of woody biomass fuels (sawdust, bark, waste

wood, and MDF board) in a lab-scale packed bed batch reactor,

indicates the fraction of the volatile-N increased with increasing

fuel-N content [95].

The mass ratio of the released NO x precursors (e.g., NH3, HCN,

HNCO, NO, N2O, and NO2) from the biomass bed under grate-firing

conditions depends on, for example, the type of the biomass fuels

[95,101], pyrolysis temperatures [102], fuel N content, stoichio-

metric air ratio, particle sizes, and moisture contents [95]. The

most relevant NO x precursors may include NH3, HCN, HNCO, and

NO, amongst which NO is the major NO x precursor from the fuel

bed under air-rich conditions, while NH3 is the most important

NO x precursor under fuel-rich conditions [11,95,106]. Table 6 lists

some of the latest efforts involving NO x precursors released frombiomass bed on the grate: either experimental study of the release

characteristics of NO x precursors from the biomass bed, or

modelling of NO x formation and emissions from fixed-bed or

grate-fired boilers on the basis of a certain NO x precursors. For the

latter, good agreements between the prediction and the measure-

ment were reported [54,57,107].

The experimental results showed the conversion of fuel-N to

gaseous N species was strongly dependent on the stoichiometric

air ratio in the fuel bed lfuel bed, the fuel-N content, and the kind of

biomass fuels. In the experimental data, conversion rates of NO,

NH3 and HCN were correlated to different values of lfuel bed [95]:

ui ¼ kilfuel bed þ di ½ (1)

in which u i represents the conversion rate of i-th N species (e.g.,NO, NH3 and HCN). ki is the slope, and di the y-intercept. lfuel bed is

ARTICLE IN PRESS

Fig. 7. The fuel-N conversion pathways.




the local stoichiometric air ratio in the fuel bed and is given by

lfuel bed ¼ nO;available

nO;stoichiometric½ (2)

where nO,available and nO,stoichiometric represent the local amount of

total oxygen available in the fuel bed and the oxygen needed for a

complete combustion of the fuel, respectively. The coefficients (ki

and di) for the NO x precursors NO, NH3, and HCN, together with

the fuel-N content and the mean particle size of the biomass fuels

investigated, are given in Table 7.

These kinds of models are useful in providing the subsequent

freeboard CFD modelling with the boundary conditions required

(i.e., the profiles of the NO x formation precursors, e.g., NO, NH3,

and HCN) for the predictions of NO x emission from industrial

grate-fired boilers burning biomass as well as for the developmentof design guidelines for low-NO x biomass grate furnaces.

The primary measure for NO x reduction is air-staged combus-

tion, which has also proven to be useful for grate-fired boilers

burning biomass [108]. Unlike char-N, volatile-N is amenable to

reduction to N2 through inexpensive techniques such as burner

and air flow configuration modifications, which can reduce NO x

emission by 50–80%. The most economical combustion modifica-

tion, to reduce NO x, is air staging. Staged combustion limits the O 2

availability in the flame and reduces peak flame temperatures to

some extent to reduce NO x formation, and produces a fuel-rich/

fuel-lean sequence which is favourable to the conversion of fuel-N

to N2. The secondary measures for NO x control, e.g., selective

catalytic reduction (SCR), could be less attractive for grate-firing of

some biomass fuels, because of the accelerated deactivation of thecatalysts caused mainly by the potassium salts that are present in

the submicron ash (dpE100nm) [71,109–111]. Co-firing biomass

(bagasse, wood chips, sugarcane trash, and coconut) with

bituminous coal in a 18.68 MWe travelling grate boiler was found

to have the capability to reduce both NO x and SO2 from the

existing coal-fired power plants [42,112], in which the biomass

ash could play some roles in the NO x reduction. The spraying of an

aqueous solution of sulphate, (NH4)2SO4, into the hot flue gases

upstream of the superheaters, which was originally proposed to

reduce the deposition rates and the corrosion rates for super-

heater tubes, can also drastically reduce the NO x emissions

[113–116].

4.1.3. HCl and SO x emissions and control

During grate firing conditions, the Cl contained in the biomass

mainly forms gaseous HCl, or alkali chlorides (e.g., KCl and NaCl),

and S forms mainly gaseous SO2 and alkali as well as alkali earth

sulphates [27,117]. Due to the subsequent cooling of the flue gas in

the boiler, a large part of the Cl condenses as salts on the heat

exchanger surfaces or on fly ash particles in the flue gas. The main

effects of Cl are the corrosive effect of chloride salts and HCl on

the metal surfaces in the boiler [10,118–120], acidic pollutant

emissions (e.g., HCl) and particulate emissions, and the effect of

HCl on the formation of PCCD/PCDF [121]. SO x forms sulphates

and condenses also on the heat transfer surfaces or forms fly ash

particles, or reacts directly with fly ash particles deposited on heatexchanger surfaces (sulphation) [27]. In short, the release of the

relevant inorganic elements (e.g., S, Cl, and K) is not only

important for direct pollutant emissions (e.g., HCl and SO x), but

also very closely related to the deposition, corrosion, and erosion

problems. The experimental studies, conducted at conditions that

resemble the local conditions on the grate in biomass-fired grate

boilers [117,122–127], indicated that the detailed release char-

acteristics of the inorganic elements depended on the type of the

biomass, the combustion temperatures, the ash composition, the

biomass char matrix, and so on.

The emissions of HCl and SO2 from biomass grate-firing can be

controlled by different means. The most common measure used in

power plants is the post-combustion flue gas cleaning systems,

e.g., by scrubbing the flue gas with limestone or by dry-sorptionwith Ca(OH)2. HCl and SO2 emissions can also be reduced, to a

ARTICLE IN PRESS

Table 6

NO x precursors released from biomass bed under grate-firing conditions

Subject NOx precursors released from the fuel bed on a grate

Modelling of NO x formation from fix-bed combustion of straw [99]. (1) The fuel-N released into volatiles, char and directly converted to NO was

estimated to be 71/25/4 based on [101].

(2) The volatile-N was assumed to form NH3/HCN/HNCO with the mass ratio of

90/5/5 based on [100]. (3) All the char-N was converted to NO.

Modelling of NO x formation from an industrial wet wood chips-fired grate boiler

[57] and a small-scale wood pellet grate boiler [107].

(1) All the fuel-N was assumed to be released into the volatiles. None of it was in

char or directly converted to NO.

(2) The volatile-N was assumed to be released in the form of NH3 and HCN with

a molar ratio of 50/50.

Modelling of straw combustion in a 38 MWe grate boiler, with NO emission as a

main issue [54].

(1) Fuel N was split into volatiles and char.

(2) NH3 was assumed to be the only precursor for the volatile-N.

(3) Char-N was oxidized to NO.

Experimental study of N species release from a broad range of woody biomass

fuels in a lab-scale packed bed batch reactor, in order to provide a

subsequent CFD gas phase combustion model with the boundary conditions

required for NO x emission prediction and reduction [95].

(1) NO2 and N2O had very low concentrations. HNCO was not studied due to the

lack of reference data for the FTIR equipment. The main NO x precursors were

NH3, HCN and NO.

(2) A release model was derived as shown in Eq (1).

(3) The total conversion rates of the different N species were predicted

accurately by the model. Most importantly, the major release zone (pyrolysis

& gasification) of the biomass fuel bed was well predicted.

Table 7

Derived parameters for the release functions of the relevant N species [95]

Fuel Fuel-N (wt%

d.b.)

Mean dp

[mm]

kNO dNO kNH3 dNH3

kHCN dHCN

Sawdust 0.06 0.3 3.30 1.88 0.62 0.91 0.49 0.49

Bark 0.27 3.0 0.85 0.44 2.12 1.94 0.15 0.12

Waste

wood

1.00 2.8 0.90 0.70 0.91 1.20 0.03 0.04

MDF board 6.87 2.7 0.15 0.10 0.94 0.94 0.06 0.03




smaller extent, by dry-sorption with fly ash on baghouse filter

surfaces.

Pre-treatment of the biomass fuels before being fed to the

boiler, for instance removal of most Cl and K and some S by

aqueous leaching process [128], can reduce HCl and SO2 emissions

and mitigate the deposition and Cl-associated corrosion. However,

leaching process would add moisture to the biomass fuels, which

may cause feeding or other operating problems that could beequally onerous. Such a process will also increase overall costs for

fuel handling and fuel preparation.

The sulphur-retention promotion by in situ sorbent addition

for biomass combustion was found to be possible: placing the

sorbent on top of the fuel bed on the grate, in the path of the

evolving SO2, was sufficient to get much of the effect obtained

when the fuel and sorbent were well-mixed. However, the rule of

thumb developed for optimum sorbent addition for in situ coal

combustion desulphurization, Casorbent/Sfuel ¼ 2, was found to be

insufficient for biomass fuels [41]. This high Casorbent/Sfuel required

may be expected. Most probably, it is because of the reducing

conditions in the fuel bed on the grate, which do not really favour

calcium-sulphur reactions. A fine control of the pyrolysis

temperatures on the grate may also reduce the Cl and S releasedinto the gas phase [122].

4.1.4. PCDD/PCDF emissions and control

PCDD/PCDF can be formed in very small amounts from all

biomass fuels containing Cl. An estimation of the emissions of

PCDD/PCDF into the air for Austria, Germany, Japan, the Nether-

lands, UK, and USA shows that the major contributors are

incineration of municipal, hazardous, and hospital wastes [129].

The formation mechanisms and the control of PCDD/PCDF

from biomass combustion are well elaborated elsewhere [130].

There are probably three primary routes for PCDD/PCDF forma-

tion: gas-phase reactions involving chlorinated precursors; con-

densation reactions involving gas-phase precursors and fly ash;

and solid-phase reactions on the surface of fly ash involving metal

chlorides and fly ash carbon (i.e., the so-called de NOVO

synthesis). However, PCDD/PCDF formation is predominantly

associated with heterogeneous reactions involving fly ash. These

low-temperature synthesis reactions can occur downstream of the

combustor at temperatures ranging from approximately

250–600 1C [130], or only in an even narrower temperature

window of 250–350 1C since the dioxins can be destroyed at

higher temperatures [131,132]. In addition to chlorine (Cl),

activated carbon, oxygen, and catalysts (e.g., CuCl2, CuO, CuSO4,

Fe2O3, ZnO, NiO, Al2O3, amongst which CuCl2 is the most reactive

one) are necessary for PCDD/PCDF formation [27,130].

The primary measures to control PCDD/PCDF emissions are

from combustion technology effects. Combustion conditions and

the time/temperature profile in the cooling zones downstream of the combustor determine the potential for PCDD/PCDF formation

within the devices. The PCDD/PCDF emissions can be significantly

lowered by reducing the entrainment of incompletely burned fly

ash particles, by ensuring a complete combustion of the flue gas

and a complete burnout of the fly ash particles (o0.5% carbon

content preferred) at low excess air ratios, as well as by lowering

heavy metal contents in the fly ash particles. For instance, a linear

increase in the grate/lower furnace temperature was found to

produce an exponential decrease in PCDD/PCDF emissions from

salt-laden wood waste-fired grate boilers [121], which could

partly be attributed to the effects of the increased combustion

temperatures on flue gas combustion and fly ash particle burnout.

Other measures include dry sorption with activated char or

catalytic converters [6,27]. The field experience in the MSW,cement, and hazardous waste industries shows that the most

effective mode of control may be to limit the temperature

entering the particulate control device to values lower than the

optimal formation window [131,133,134].

4.1.5. Particulate matter and heavy metals emissions and control

Formation and emissions of particulate matter from biomass

combustion have drawn considerable attention because of theconcern that they may contain toxic elements or species of

relatively high concentrations, and have been a key research area

in biomass combustion; for example, under the EU-supported

projects BIOASH and BIOAEROSOLS [126,135]. The main focus of

the current R&D activities on this issue includes the following

aspects: (1) characterization of aerosols and ash particles (e.g.,

chemical composition, size distribution and morphology, concen-

tration in flue gas), which may depend on the type of biomass

fuels, firing technology, operation conditions; (2) fundamental

mechanisms of the formation and growth of fly ash particles and

aerosols including the release behaviour of ash-forming elements,

and how fly ash particles and aerosols contribute to deposition

and corrosion; (3) how to effectively reduce their emissions, for

instance, by improving aerosols separation efficiencies of flue gascleaning devices, and by using certain additives into biomass-fired

boilers; and (4) the partition and distribution of metals in bottom

ash and fly ash.

The existing data, concerning ash formation and emissions

from biomass combustion under grate-firing conditions or fixed-

bed conditions, are comparatively limited [110,126,135–143]. Most

of the knowledge is retrieved from measurements in large-scale

circulating fluidized bed (CFB) boilers, some of which are briefly

reviewed in [110]. The biomass fuels fired in a CFB boiler and a

grate-fired boiler have substantially different time-temperature

histories. Because of the intensive mixing in the dense bed in a

CFB boiler, the biomass fuels will be quickly dried, heated, and

combusted. The bed temperature in CFB combustion is relatively

low, about 800–900 1C. This is very different with biomass

conversion process in a grate-fired boiler, where biomass fuels

are slowly heated, dried, and pyrolysed as they are transported on

the grate, and finally the char will be combusted at high

temperatures, about 1000–1100 1C (see Fig. 5 for this process).

These differences will significantly affect the volatilization of

particle-forming precursors.

The ash formation during biomass combustion under grate-

firing conditions is illustrated in Fig. 8. Part of the inorganic

elements contained in the fuel may be released and form

inorganic gas species and particulate matter (fly ash particles

and aerosols), whilst the remaining part forms bottom ash. It can

be seen, from the figure, that the particulate matter can be formed

by two different modes, which lead to a characteristic bimodal

particle size distribution [110,136,137,142,144–146]. One is the fine

mode, in which the main route of particle formation is nucleationand condensation from the gas phase. The fine mode usually

consists of aerosols with particle diameters between 30 and

300 nm. The other is the coarse mode, which mainly consists of

non-volatilized ash residuals and results in fly ash particles with

diameters of 1 mmodpo10mm. Similar ash formation mechanisms

could also be found in [135,145–151].

Amongst all the ash streams, bottom ash represents the

majority, in terms of mass fractions, for grate-fired boilers burning

biomass. The mass fraction of the bottom ash varies with different

factors, e.g., the type of grate-fired boilers, the operation, and the

fuel properties. In a 10.7 MWfuel grate-fired boiler burning straw

which produces 2.3 MWe and 7 MJ/s heat, it was estimated that

approximately 80% of the ash ended up as bottom ash [152]. In a

108 MWfuel grate-fired boiler burning wheat straw equipped withwater-cooled vibrating grates, the bottom ash amounted about

ARTICLE IN PRESS




85–90% of the total ash as a result of reduced primary air flowrate

[53]. Bottom ash and fly ash were sampled from 19 MSW-fired

grate boilers, whose capacities ranged from 60 2 to 500 3ton/

day. The mean mass fraction of bottom ash for the 19 boilers is

about 77% (with 60% as the minimum and 88% as the maximum)

[138]. Concerning the mass split between the aerosols and fly ash,

there are also some investigations, for instance [110,150]. The

experiments on two moist forest residue-fired grate boilers (1 and

6 MWth in capacity, respectively) show that small changes in the

boiler operating parameters can have a large influence on the

mass split between aerosols and fly ash. For instance, PM110 (i.e.,

particulate matter with diameters in the range of 1–10 mm) mass

concentrations increased by more than one order of magnitude

when the boiler load was increased from 50% to 75%. The coarse

(41mm) and fine mode (o1mm) masses were of equal magnitude

downstream of the cyclone when the boilers were operated at

higher load. At lower load, PM1 (o1mm) dominated PM10

(o10mm) [110].

Under grate-firing conditions, the most volatile metals con-

tained in biomass fuels (e.g., Hg, Th, and Se) are completely

vaporized and then may be released into the atmosphere with the

flue gas or condensed on the surfaces of aerosols and fly ash

particles. Measurement data from 19 grate-fired incinerators

burning MSW showed that lithophilic metals such as Fe, Cu, Cr,

and Al remained mainly in bottom ash whilst volatile Cd

transferred to fly ash. Two-thirds of Pb and Zn remained inbottom ash despite their relatively high volatility [138]. The

generally non-volatile compounds (consisting mainly of refractory

species such as Ca, Mg and Si) and some bound volatile

compounds (e.g., K, Na) usually remain on the grate and form

the usable bottom ash [26,27]. So, the bottom ashes from grate-

fired boilers burning biomass are comparatively less problematic

in terms of the leaching concentration of toxic elements and may

be directly used on agricultural land or in forests as secondary raw

materials with fertilizing and liming effects, or landfilled [27].

Compared to the bottom ash, the aerosols and fly ash particles

are more likely to cause some problems. Firstly, they have a

considerable influence on the formation of slagging and fouling

deposits on heat transfer surfaces in grate-fired boilers, which

lead to reduced heat transfer and corrosion. Secondly, the aerosolscontribute to the ambient air pollution level of particles, which

are shown to have adverse health effects on humans. For instance,

it was found that inhaling aerosols emitted from biomass

combustion causes significantly more lung damage in mice than

do aerosols from coal, probably because of the zinc content in the

aerosols from biomass combustion, from which it was even

suggested the environmental advantages of biomass combustion

may be tempered by the risk to respiratory health [153]. The

formation mechanism of aerosols and fly ash particles under

grate-firing conditions may be referred to Fig. 8. The character-

izations of the aerosols and fly ash particles from grate-fired

boilers burning representative biomass fuels, e.g., woody biomass,

agricultural residues (straw), and MSW, are given in Table 8. It

should be emphasized that these measured data from field studies

are just for reference. These data are quite dependent on the

capacity, type and operation of the boiler, the physical and

chemical properties of the biomass fuel fired, and the detailed

combustion control technology (e.g., temperature, turbulence, and

residence time).

There are also some modelling efforts in examining possible

practical influences on aerosol formation in biomass-fired boilers

and determining the amount and chemical composition of

particle emissions, such as the model proposed for the formation

of gaseous alkali sulphates [154] or model for aerosol formation

[135,139].

Since aerosols contribute to the ambient air pollution level of

particles and may contain toxic elements, the effective reductionin aerosols emission is necessary in order to boost biomass

combustion. There are some commercially available dust-pre-

cipitation technologies, as seen, for example, in [155], which

include cyclones, multi-cyclones, flue gas condensation units,

electrostatic precipitators (ESP) (including wet ESP and dry ESP),

and baghouse filters. In general, baghouse filters have the best

precipitation efficiency for aerosols; wet ESP and dry ESP also

have good precipitation efficiencies; scrubber condensers are less

efficient in capturing aerosols; and cyclones are suitable only for

coarse fly ash particles. Besides the dust precipitation, reduction

in aerosols emission may also be achieved from their formation

routines. The influence of six sorbents on the formation of

aerosols during straw combustion in a 100 MWfuel grate-fired

boiler was studied in full-scale. The observations show asignificant reduction in the aerosols formation by using five of

ARTICLE IN PRESS

Fig. 8. Schematic illustration of ash formation routines during grate-firing of biomass.




the six sorbents studied, i.e., bentonite, ICA5000, clay, mono-

calcium phosphate, and ammonium sulphate [143].

4.2. Deposit formation and corrosion

Deposition (i.e., slagging and fouling) and corrosion problem is

one of the major issues that play an important role in the design

and operation of a combustion system. In solid fuel combustion,

the particulate matter formed during combustion may be

deposited on furnace walls and heat-exchanger tubes, which will

reduce the heat transfer and could also give rise to corrosion

problem. Biomass-fired furnaces, in particular straw-fired fur-

naces, are often reported to have severe deposition and corrosion

problems compared to coal-fired boilers (see Fig. 9 for examples).

4.2.1. Deposition indices based on fuel properties

Deposition indices are always an important concept whenevaluating the deposition potential of a solid fuel or handling

deposition problems during combustion of solid fuels. There are

many deposition indices for the prediction or estimation of the

ash deposition tendency, which are based on the fuel properties,

particularly the fuel ash chemistry. The commonly used deposi-tion indices include base/acid ratio (B/ A ¼ (Fe2O3+CaO+MgO+

K2O+Na2O)/(SiO2+Al2O3+TiO2)), iron index (Fe2O3 (B/ A)), slagging

factor (dry S% (B/ A)), silica ratio (100SiO2/(SiO2+Fe2O3+CaO+

MgO)), critical viscosity at 1426 1C (CV1426 1C in the unit of poise),

estimated ash temperature corresponding to an ash viscosity

of 250 poise (T 250 in the unit of 1C), alkali index—the ratio

of the total amount of sodium and potassium expressed as

their corresponding oxides to the heating value of the fuel

(ðY aK2O þ Y aNa2OÞ=Q F in the unit of kg/GJ), chloride–sulphate molar

ratio ((Cl+2S)/(K+Na)), multi-fuel fouling index—the percentage of

water-soluble alkali and alkaline-earth metals (given as oxides),

ash melting points (initial deformation temperature, hemisphe-

rical temperature and flow temperature) [157]. Most of these

indices were originally proposed for coals, and different limitsare suggested for coal quality parameters relating to deposition

ARTICLE IN PRESS

Fig. 9. Deposits on super-heaters during firing straw or straw/coal in boilers. (a) Deposits on superheater in upper furnace during firing straw at Masnedø CHP plant [146].

(b) Deposit build-up on superheaters after 1 week of co-firing coal and straw at Amager power plant [156].

Table 8

Characterization of aerosol particles (o1mm) in flue gas or fly ash particles

Grate-fired boilers Sampling point Concentration of

aerosols in flue gas

Particle size distribution of

aerosols (or fly ash particles)

Composition of aerosol particles

(or fly ash particles)

25MWfuel wheat straw-

fired grate boiler

[136]

The flue gas particles are

sampled upstream the

electrostatic precipitator

(ESP), where gastemperature is 120 1C

and O2 concentration

fluctuates 8%.

The concentration of the

aerosols is 300–480 mg/

N m3 (cascade impactor)

or2 106–1.8 107 N cm3

(scanning mobility

particle sizer).

The number geometric mean of the

submicron peak is 0.19–0.30mm.

Small particles (0.1–0.3mm) are

close to spherical. Large particles(0.3–0.8mm) vary from almost

spherical to aggregates consisting

of 5–10 distinct primary units with

dp 0.1–0.2mm.

The aerosol particles mainly

consist of KCl and K2SO4 with

minor amounts of P. Elements Ca,

Mg and Si are detected in largerparticles only (41 mm).

6 MWth moist forest

residue-fired grate

boiler at 85% load

(The dominant

species of the fuel

are spruce and pine.

The moisture content

is 40%.) [110,137]

The flue gas particles

were sampled

downstream the cyclone

and upstream the ESP,

where the gas

temperature is about

1901C.

The concentration of the

aerosol particles is close

to those of the coarse

particles (1–10mm): both

are 79mg/Nm3

(related to 13% CO2 dry

gas).

Bimodal particle size distribution is

detected: aerosol particles have a

peak at about 0.2 mm; while coarse

particles are peaked at 2 mm.

The dominant species (mass ratio

410%) in the aerosol particles are

K, S, and Cl. Minor elements are Zn

and Ca. Cr, Fe, Ag, Cd, Mn, P, etc. are

present as traces. The dominant

species in the coarse particles are

Ca, K, and S.

Waste wood fired grate

boiler (nominal

capacity 40 MWth)

[135]

From the flue gas duct

behind the economizer

(mean flue gas

temperature 1781C).

The mean concentration

of aerosols is 84.2mg/

N m3 (related to 13 vol.%

O2 and dry gas).

The aerosol particles are mainly in

the range of 0.088–0.707mm with

the peak at 0.354mm.

The aerosols mainly consist of Cl, K,

and Pb. Ca, Na, S, Si, and Zn are

present in minor amounts. Fe, Mn,

and P are present as traces.

450 2 tonnes/day MSW(90% household

waste and 10%

business waste)

grate incinerator

[140]

Fly ash was sampledfrom the ash pit under

the bag filter, which is

located just before the

stack.

No particleconcentration data.

Over 95% of the fly ash particleswere o149mm, in which 49.4%

were in between 53–75mm. The fly

ash is characterized by a more

uniform distribution of concaves

and agglomeration on its surface.

The fly ash has highly complexmineralogy. The main crystalline

compounds detected include KCl,

NaCl, and SiO2. The fly ash requires

further treatment before final

disposal since the leaching

concentration of Pb exceeds the

regulatory level.

In this table, N m3 and Ncm3 mean m3 and cm3 under normal conditions (1 atm and 273K), respectively.




potential. As an example, the coal slagging potentials can be

evaluated online by calculating some of the above deposition

indices from its ash analysis [158]. Care must be taken with these

indices and their application.

The capability and accuracy in predicting ash deposition

potential. The fuel properties represent only half the depositionstory, and the boiler design also plays a large part in

the occurrence of ash deposition problems. Generally, if the

combustion temperatures in the furnace are high, or the

aerodynamics encourage flame impingement, then the potential

for ash deposition problems will be high. On the other hand, if

the fuel ash chemistry is conducive to the formation of sticky

compounds at the prevailing furnace temperatures, then the

potential for deposition problems will also be high [157]. So the

different indices only indicate an ash deposition tendency. In

addition, the limits for the deposition indices, suggested in the

literature, are not necessarily valid for all fuels. For a specific fuel,

one may have to experimentally determine the limits.

The different indices could lead to inconsistent deposition

tendencies for a same fuel. The alkali index and multi-fuelfouling index, both of which have been suggested to be also

applicable to biomass fuels and biomass mixture, were

calculated for rice husk, rice straw, and eucalyptus bark

samples, as well as for some other husks and hulls, straws,

and Scandinavian barks. The calculated indices showed

different deposition risks, in particular for rice husk: the alkali

index indicated a certain risk of deposition whilst the multi-

fuel index showed a low or non-fouling risk. The measure-

ments showed that rice husk was a non-fouling fuel [159].

The correct understanding and use of the ash melting points.

Biomass fuels contain a relatively high content of K and Na,

which are believed to significantly lower the melting points of

ash, and hence may increase ash deposition and fouling

tendency [13,23]. However, in many cases the standard melting

point measurements have resulted in findings that do not

directly correlate with the ash behaviour in full-scale combus-

tors [159]. One of the possible reasons could be the measure-

ments or the understanding of the melting point. Inorganic

mixtures such as fuel ashes do not have one sharp melting

point. Instead, they melt stepwise in a temperature range

where the difference between the temperature of the first

appearance of melt (T 0) and the temperature of complete

melting (T 100) can be several hundred degrees. It was found

that the amount of a melt present in the condensed phases,

rather than the composition of the ash itself, was the main

reason for ash deposition problems. In order to deposit on a

surface, an ash particle should contain a certain amount of

liquid melt. Based on experience with different kinds of boilers

or combustors, it was suggested that an amount of 15 wt% of the condensed phases molten at a certain temperature enabled

deposit formation in the flue gas channel [160–162]. The

melting curves (i.e., the melt portion as a function of the

temperatures) can be used to determine the characteristic

temperatures: the sticky temperature (T 15), i.e., the tempera-

ture below which the salt mixture contains less than 15 wt%

molten phase, and the flow temperature (T 70), i.e., the

temperature above which the share of the liquid phase is

larger than 70 wt%. With this approach, it was assumed that

the material did not stick on surfaces at flue gas temperatures

below the sticky temperature of the depositing material [163].

This approach was used to predict the fly ash deposition (bed

sintering tendencies as well) for different solid fuels (coal and

biomass), and good agreement between the predictions andthe measurements was found [162].

4.2.2. Mechanisms of deposit formation and high temperature

corrosion

To probe into the fundamental mechanisms of fly ash anddeposit formation and corrosion in grate-fired boilers burning

typical biomass fuels (straw, as well as woody biofuels and MSW),

a significant amount of full-scale measuring campaigns, as well as

a few lab- or pilot-scale studies have been conducted, for example,

[69,70,164–171]. Generally speaking, both the volatile (alkali)

inorganic vapours and the inert non-volatile mineral matter can

contribute to the formation of deposit on heat transfer surfaces in

the boiler, in which they may play different roles in the build-up

of the deposit, to be discussed later. The corrosion mechanisms in

combustion systems include gaseous Cl-species induced corro-

sion, solid-phase reactions involving Cl-species in deposits,

reactions involving molten Cl-species, molten sulphate corrosion,

and so on, as reviewed in [10]. Which mechanism dominates will

depend on the combustion environment, combustion tempera-ture, metal temperature, and also the presence of elements such

as alkali metals, sulphur, silicon, and aluminum. Fig. 10 sketches

the in-deposit sulphation corrosion mechanism occurring in a

waste incineration plant [172]. Potassium species are supposed to

behave similarly to the sodium species. This mechanism was also

believed to be responsible for the most severe corrosion problems

on the superheater tubes in grate-fired boilers burning some other

biomass fuels (e.g., straw) [10,119]. Molten phases increase the

corrosion rate because of the faster chemical reactions in a liquid

phase and the electrolyte or pathway provided by the liquid phase

for the electrochemical attack [173].

The following sections briefly describe the deposition forma-

tion and high-temperature corrosion in grate-fired boilers burning

a few representative biomass fuels, i.e., straw, woody biofuels, andMSW, respectively.

Straw-fired grate boilers: The findings are summarized in [119].

Composition of fly ash and deposits: The fly ash and deposits

are rich in KCl (40–80 wt%).

Deposit formation: Initially on the clean tube, inorganic

vapours and fine particles that arrive at tube surface in a sticky

condition contribute to the initial deposit formation, and they

deposit on the entire circumference of the tube. After that, inertial

impaction of big fly ash particles contributes to the deposit build-

up, but mainly on the upstream side of the tube. So the deposit

formed consists generally of a white dense inner layer covering

the entire circumference of the tube and an ellipse-shaped depositon the upstream side, as sketched in principal in Fig. 11. The main

ARTICLE IN PRESS

Fig. 10. Schematic presentation of solid-phase reactions involving Cl-species in

deposits [172].




deposit on the upstream side consists of fly ash particles and large

amounts of condensable salts, forming a matrix that glues the fly

ash particles together. The deposit is quite porous, which makes it

an effective insulator [156]. In the deposit the fly ash particles are

dominated by K and K–Ca silicates.

High-temperature corrosion: the relatively high partial pres-sure of HCl in the bulk gas in a furnace will probably not cause

severe gas-phase corrosion attacks [10] under oxidative condition.

Instead, the corrosion of superheater tubes is closely connected to

the deposit chemical composition, in particular the composition

of the inner part of the deposit. A characteristic feature found in

deposits is the presence of a dense inner layer of KCl or K2SO4 with

a structure of iron oxide (Fe xO y) in it. A suggested corrosion

mechanism for chlorine corrosion is based on gaseous Cl attack,

where Fe and Cr in the metal react with gaseous Cl, forming

volatile metal chlorides. The high partial pressure of chlorine close

to the metal is believed to be caused by a rapid sulphation of KCl

to K2SO4 in a melt, formed adjacent to the metal surface, as

depicted in Fig. 10, by replacing the sodium species with the

potassium species. This mechanism can explain the shift in

corrosion behaviour with temperature observed in full-scale

corrosion tests (i.e., selective corrosion: negligible at steam

temperatures of 4501C, significant if the metal temperature is

raised above 520 1C). At low metal temperatures, the solid-phase

sulphation is slow, and the metal suffers only from general

oxidation. When the KCl/K2SO4/Fe system becomes molten, the

KCl sulphates quickly in the melt, thereby generating a high

partial pressure of Cl2/HCl. Subsequently, this causes accelerated

oxidation and possible internal corrosion of the metal. The K2SO4

found in deposits in straw-fired boilers is believed to originate

mainly from deposition of gaseous KCl, followed by a subsequent

slow sulphation of KCl in the solid phase [119].

However, this suggested corrosion mechanism may not be

universal, particularly for the initiation of the corrosion. There are

quite some recent results indicating that sulphation is not needed

for the chloride corrosion to be initiated in the super-heaters and

that pure KCl will also cause heavy corrosion even in a gas

atmosphere with no SO2. For example, the influence of KCl on the

oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel

at 600 1C in 5%O2 and in 5%O2+40%H2O is investigated in the

laboratory. Based on the observations, it was proposed that the

rapid corrosion was initiated by the reaction of potassium chloride

with chromium oxide in the scale, forming potassium chromate

and Cl2 or HCl [174]:

Cr2O3ðsÞ þ 4KClðsÞ þ 52O2ðgÞ32K2CrO4ðsÞ þ 2Cl2ðgÞ (3)

Cr2O3ðsÞ þ 4KClðsÞ þ 2H2OðgÞ þ

3

2O2ðgÞ32K2CrO4ðsÞ þ 4HClðgÞ (4)

Woody biofuels-fired grate boilers: three different aerosol

formation processes are active, depending on the woody fuels

fired [119,150], which can also be seen in Fig. 8.

When firing chemically untreated wood chips, aerosol forma-

tion is comparable with aerosol formation in straw combustion:

Nucleation and condensation of alkali-salts and subsequent

coagulation of particles are the dominating mechanisms.When firing waste wood, Zn becomes more important. Under

reducing conditions in the fuel bed, Zn is released to the gas phase

and oxidized to ZnO, which then forms the first nuclei. This is

supposed to happen right after the flue gas leaves the fuel bed.

Subsequently, further nucleation of alkali metal and heavy metal-

salts is suppressed by condensation on the large specific surface

provided by a high number of ZnO nuclei, and particles grow due

to condensation and coagulation.

When firing bark, the aerosol formation process is somewhere

in between the two mechanisms mentioned above. The sub-

micron Ca-particles entrained from the fuel and ZnO-particles

formed according to the mechanism mentioned above normally

do not provide the specific surface area needed to suppress

nucleation, so that K-salts still form new particles. Subsequently,the particles grow by condensation and coagulation.

MSW-fired grate boilers: The findings of mature deposits in

MSW-fired grate boilers are summarized [119]. When going

from the outer to the inner deposit layers, a decrease in [Ca],

[Cl] and [Si] and a simultaneous increase in [S], [K] and [Zn] are

observed. The chlorine is responsible for the most serious

corrosion, and the decrease in Cl in the inner layer is because

the chlorides are converted to sulphates. The in-deposit

corrosion mechanism [172] suggested is sketched in Fig. 10.

Stronger sintering in the outer deposit layers is observed and

significant porosity is found in the inner deposit layer.

4.2.3. Possible solutions to the problems of deposition and high-

temperature corrosion

Both high-temperature corrosion and deposit formation during

biomass combustion can be mitigated by using additives, or by co-

firing with, for instance, coal, peat or sludge. The high-tempera-

ture corrosion can also be mitigated by using new alloys or new

forms of ceramic composite coating, or by reducing the surface

temperatures of super-heaters.

The first solution is the use of additives, to raise the melting

temperatures of the ash formed during biomass combustion, or to

prevent the release of gaseous KCl, or react with KCl to form less

corrosive components, or to combine the effects. The lower the

melting temperatures of the ash and/or the more vaporizable the

alkali or chlorine compounds, the higher will be the risk for ash-

related problems. Raising the ash melting temperatures canlargely increase the potentials for the use of the biomass fuels.

The materials that have been found to raise the melting

temperatures of ash, to temperatures higher than those normally

encountered in boiler furnaces, include Al2O3, CaO, MgO,

CaCO3 MgCO3 and kaolin [7,175]. The effect of the additives is

to enrich the ash formed during combustion with non-potassium/

sodium compounds. For example, addition of 3 wt% of kaolin to

chopped oats straw can raise the deformation temperature of the

ash from 700 to 1200–1280 1C [176]. However, one could argue in

a different way, as to how the additives raise the melting or

deformation temperatures. Additives do not change the first

melting temperature (T 0) at all. They may dilute the ash and thus

decrease the percentage of the molten phase in the mixture,

which could show as an increase in the measured empiricaltemperature for the radical deformation of a standard body

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Fig. 11. Principal sketch of the wheat straw-derived deposits formed on top of the

probe [69,119].




pressed from the ash particles. Different additives have been

applied, either to the flue gas or with the fuel, and proven useful in

commercial biomass-fired boilers. For instance,

ChlorOut (ammonium sulphate), a concept developed and

patented by Vattenfall AB [177], may be one of the most

attractive methods for different combustion technologies,

since the additive is added to the flue gas in a certaintemperature window. An aqueous solution of ammonium

sulphate, (NH4)2SO4, is sprayed into the combustion zone at

temperatures around 800–900 1C upstream of the superhea-

ters. It effectively converts alkali chlorides (e.g., KCl) into alkali

sulphates (e.g., K2SO4). These sulphates are much less corrosive

than the chlorides and therefore the overall corrosion rate is

reduced. The spraying of ammonium sulphate can also reduce

NO x formation. The main reactions involved in ChlorOut

process include

ðNH4Þ2SO4ðaqÞ ! 2NH3ðgÞ þ SO3ðgÞ þ H2OðgÞ (5)

2KClðgÞ þ SO3ðgÞ þ H2OðgÞ ! K2SO4ðsÞ þ 2HClðgÞ (6)

4NH3ðgÞ þ 4NOðgÞ þ O2ðgÞ ! 4N2ðgÞ þ 6H2OðgÞ (7)

ChlorOut has been successfully tested in a number of biomass-

fired fluidized bed boilers. The results showed that ammonium

sulphate reduced the KCl levels in the flue gases, removed the

chlorides from the deposits and the metal/oxide interfaces, greatly

reduced the deposition rates and halved the corrosion rates for

superheater materials [114,115]. ChlorOut has also been tested in

grate-fired boilers burning biomass [113,116]. The tests in the

grate-fired boiler at the waste incineration plant Mullverwertung

Borsigstraße (MVB) in Hamburg, within an EU co-financed project

NextGenBioWaste, showed a marked reduction of the corrosivealkali chlorides when ammonium sulphate was injected. The

amount of deposit build-up was halved and the corrosiveness of

the deposits was reduced. There were also some environmental

benefits to using the additive: the level of NO x in the flue gas was

drastically reduced and a reduction in the amount of dioxins in

the fly-ash was also detected. The results from the tests were used

for designing a permanent ChlorOut system in the waste-fired

grate boiler at MVB. The permanent installation was planned for

the end of 2007 [116]. In the light of the idea of ChlorOut, two

different reagents, Al2(SO4)3 and Fe2(SO4)3, were tested as

slagging and corrosion control techniques for biomass firing

[178]. It was concluded that in the following reaction (where M

stands for K or Na),

2MClðgÞ þ SO3ðgÞ þ H2OðgÞ ! M2SO4ðsÞ þ 2HClðgÞ (8)

the effectiveness of the reagents with constant sulphur mass flow was

Al2ðSO4Þ34Fe2ðSO4Þ3bðNH4Þ2SO4 (9)

This order is directly proportional to the temperature needed

for thermal destruction. (NH4)2SO4 probably decomposes faster

than the other sulphates, which then produce fresh SO3 over a

longer furnace zone. Complete decomposition to SO3 over a short

furnace zone leading to high local SO3 concentration may enhance

other SO3-consuming reactions than sulphation and reduce alkali

sulphation selectivity to SO3. With the new reagents, sulphation of

alkali chlorides dominates strongly over the other SO3-consuming

reactions, which is a significant advantage: chemicals can be

used with low dosages with small or insignificant increase of SO2

emissions [178].

The additives can also be added with the fuel onto the fuel bed.

However, the effects may be less general: they are more

dependent on combustion technologies. Different combustion

technologies, e.g., FBC and grate-firing, have very different

mixing, temperatures and combustion environments (i.e.,

oxidizing or reducing) in the fuel beds, which significantly

affect the effectiveness of the additives. One has to keep this in

mind when trying an additive in grate-fired boilers which hasbeen tested and proven efficient in other combustion technol-

ogies (e.g., FBC or suspension combustion). A test programme

in a grate-fired boiler at a CHP plant showed that it was

possible, from an operational, corrosion, and depositional

point-of-view, to co-fire Danish wheat straw with shea nuts,

wood chips, and olive stones on a grate, at the actual energy

shares, i.e., 25–30% on an energy base. In the short-term, full-

scale experiments, no serious operational problems related to

ash and deposit formation or corrosion of superheater tubes

were observed [119]. Adding kaolin to biomass particle seal

under CFB combustion conditions was found to have the

following effects: Kaolin could capture potassium and form

potassium aluminium silicates which have high melting points

and were unlikely to become liquid and stick to the surface of super-heaters; less potassium would be available for the

reaction in the furnace [86]. Since the contact between the

additive particles and flue gas potassium compounds in grate-

fired boilers is less efficient than in a CFB, one may not expect

the same effects if kaolin was used under grate-firing

conditions. When firing pulp sludge with pine bark or with

pine bark/agricultural waste mixture under FBC conditions,

aluminium silicate formation was found to dominate over

sulphation in the reduction of Cl concentration in deposits,

which then reduced the chlorine-induced corrosion of heat

transfer surfaces [118]. Again, one has to be aware of the

difference between FBC and grate-firing if the same method

was used in grate-firing boilers. Coal ash could also be used as

a kind of additives. It was found that during co-firing of straw

and coal there seemed to be a significant capture of K from the

straw, by the coal ash, which was observed in different utility

boilers. Due to the capture of K in the coal ash, only low

concentrations of KCl (o5 wt%) were observed, in the fly ash

and deposits, from the plants. It was concluded that ash

deposition and/or corrosion would not, most likely, be the

major problems during coal-straw co-firing in suspension-fired

boilers provided that a high-quality coal was applied for the

co-firing [179]. Under grate-firing conditions, the capture of K

from the straw, by the coal ash, may also occur to some extent.

The second solution is to use new alloys or ceramic tiles that are

resistant towards chlorine corrosion [166,180], especially for

modern large-scale biomass-fired grate boilers. There are somepreliminary tests on this subject. For instance, various austenitic

and ferritic steels are exposed in a water-cooled probe in the

superheater area of a straw-fired CHP plant. The temperature of

the probe ranges from 450 to 600 1C and the period of exposure is

1400 h. The rate of corrosion is assessed based on unattacked

metal remaining. A clear trend has been observed that selective

corrosion increases with respect to the chromium content of the

alloy [166]. Various forms of coating aimed at preventing

corrosion have been tested at the Siekierki CHP station in

Warsaw, Poland. A new form of ceramic composite coating is

effective in preventing corrosion and has been installed in

different boilers [180].

The third solution is to keep the surface temperatures low.

New biomass-fired grate boilers are often characterized by highsteam parameters (temperature and pressure) for high plant

ARTICLE IN PRESS




efficiencies. The increased steam temperatures raise concerns of

high-temperature corrosion as a result of increased tube surfacetemperatures. A modified Rankine steam cycle is presented as a

solution for a biomass-fired boiler of small or medium sizes, to

prevent the chlorine-induced high-temperature corrosion without

loss of efficiency in the steam cycle by fully utilizing the

permissible wall temperature limits [120].

Other measures against corrosion include co-flow superheaters

(i.e., superheater tubes are placed with axis parallel to particle-

laden gas flow in order to minimize the possibilities of particle

accumulation) and optimized combustion and process-control

technologies. Automatic heat exchanger cleaning systems, e.g.,

soot blowers, are often used to mitigate the deposits on super-

heater tubes. However, they could make the high-temperature

corrosion even worse by effectively removing the corrosion

products from the tubes while exposing them to new corrosive

fly ash deposits.

4.3. Modelling and CFD simulations for diagnosis, optimization, and

new design

Not all the relevant phenomena in a combustion system are

described and understood in full details, but CFD calculations give

an impression of reciprocal relationships. Mathematical model-

ling and CFD simulations form a helpful tool to improve the

understanding of the details, probe the problems, and optimize

the plant operation, as well as aid in a new design. Compared to

modelling of pulverized coal boilers, CFD modelling of biomass-

fired grate furnaces is inherently more difficult due to the complex

biomass conversion in the fuel bed on the grate, the turbulentreacting flow in the freeboard, and the intensive interaction

between them. Fig.12(a) sketches the sub-processes in grate-fired

boilers, i.e., thermal conversion of biomass in the fuel bed, and

primary combustion and burnout in the freeboard, which interact

with each other. Fig. 12(b) shows the most popular methodology

in modelling of grate-firing biomass, in which a separate model

is used to solve the thermal conversion of biomass in the fuel

bed, CFD is used for the freeboard simulation, and they are

coupled by the heat and mass transfer at the interface (i.e., the top

surface of the fuel bed). More precisely, the bed model provides

the inlet conditions (e.g., distribution of gas species concentration,

velocity, and temperature along the grate) for freeboard simula-

tion, whilst the freeboard simulation returns the heat flux

released from the flame and furnace walls onto the fuel layer tothe bed model.

However, there are different modelling methodologies for

grate-firing systems, for instance [3,63,181], in which the wholefuel bed is included as a part of the CFD simulation domain and

there is no need for a separate bed model to provide inlet

conditions. In this methodology, the precise size distribution of

the biomass particles fed into the boiler plays a decisive role in

final CFD simulation results.

Because it is difficult and expensive to carry out comprehen-

sive experimental studies on grate-fired boilers, many modelling

and CFD simulation efforts have been made instead. Table 9 lists

some of the representative modelling works on grate-firing of

biomass in the literature, in which the main purposes and findings

of the works are highlighted. As shown in the table, the modelling

and simulation works can be classified into five groups and the

first two groups’ efforts dominate. Most of the modelling and CFD

works have been validated, to different extents, by experimental

results, particularly the modelling works on biomass conversion

in the fuel bed on the grate.

4.3.1. Modelling of biomass conversion in the fuel bed on the grate

The processes in the biomass bed on the grate may be the most

grate-specific area for grate-fired boilers. The behaviour of

biomass conversion on the grate significantly affects the incom-

pletely burned char in the bottom ash, the distribution of the

combustibles released into the freeboard, and the precursors of

NO x, SO x, PCDD/PCDF, and particle formation. Therefore, biomass

conversion on the grate affects not only the combustion efficiency

of the grate-fired boiler but also the deposition and corrosion

tendency and pollutant emissions from the boiler. As seen in Table

9, there are quite some efforts on this issue, mainly by modelling.Three different approaches on how to model the fuel bed may be

found in literature.

Approach I . Fluent’s porous-media model is used to investigate

the solid-refuse bed on top of a roller grate. The results obtained

from this modelling are used as the inlet conditions for the

modelling of the freeboard region [201,202].

Approach II . More typically, freeboard modelling treats the fuel

bed by using inlet conditions based on experience or measure-

ments. When the combustion rate is prescribed as a function of

the position on the grate, inlet conditions (e.g., temperature,

velocity and individual species concentration) can be calculated

from the overall heat and mass balances of fuel components and

primary air, see, for example, [39,57,61,80,95,106,192,194,195,203].

The experience- or measurements-based bed models have beenproven to be quite robust and useful in studying biomass

ARTICLE IN PRESS

Fig. 12. Grate-firing of biomass and modelling methodology. (a) Biomass conversion in fuel bed, gas combustion in freeboard and their interaction. (b) Modelling concept:

coupling CFD and bed model.




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Table 9

Summary of the modelling of grate-firing of biomass: the main purpose and the main findings

Purpose of work Main conclusions or findings Reference

Group 1—Modelling of biomass conversion in the fuel bed on the grate

Aerodynamics study of chain link stoker mats by CFD,

visualization and tests to improve the under-grate primary air

(PA) distribution.

CFD is used to aid in redesign of a traditional link design which shows

improvement in PA distribution. Pollutant emission from chain grate

furnaces may be mainly attributed to poor PA distribution.

[182]

To develop a model to characterize and quantify the mixing of biomass fuels on a grate.

(1) The existing grate systems do not mix the refuse sufficiently. (2) Modelpredictions show good agreement with measurements from three 1/15

scaled models of industrial grates.

[183]

To derive methods on a statistical basis to describe quantitatively

the mixing process of a packed bed on a forward acting grate.

(1) Two methods are introduced, based on particles’ velocities and

trajectories, to quantify the mixing process. (2) The trajectory-based method

is believed to be more accurate and suitable.

[184]

To study the effect of fuel mixing on waste bed combustion in

MSW grate incinerators by model development and

experiments.

(1) Improvement of the combustion intensity by fuel mixing is observed. (2)

Control of the primary air supply could further enhance the waste

combustion.

[185]

To study the effect of particle mixing caused by grate movement

on the ignition, burning rate, unburnt carbon (UBC) in ash,

and so on.

(1) Increasing bed mixing from low to medium level significantly increases

the burning rate and reduces UBC in ash. (2) Excessive mixing may cause

significant delay in ignition or even extinction.

[186]

To evaluate the residence time of a moving bed on a forward-

acting grate by numerical approaches.

Discrete element method is applied to describe the motion of a moving bed

(e.g., its particles). Tracking particles can obtain better predictions of the

residence time of a moving bed.

[187]

To understand MSW combustion in a grate incinerator by

modelling and tests of wooden particles combustion in a

fixed bed testrig.

(1) A 1D bed model is developed. (2) Radiation in fuel bed is important in

initiating flame front and transferring heat to the cold bed. (3) Air supply

rate, LHV, and particle size are important parameters.

[79]

To develop a fuel bed model and incorporate it in CFD for

modelling of coal combustion in 60 MWfuel grate boiler.

(1) The properties of different bed zones determine the conditions in the gas

phase above the bed. (2) The channelled flows in furnace do not seem to be

sensitive to the details of bed model.

[188]

To develop a bed model on a travelling grate by incorporating

sub-process models and solving governing equations for gas

and solid.

(1) A 2D bed model (FLIC) is developed. (2) Model predictions without

considering channelling effect agree with experiments in total mass loss but

show big discrepancy in temperature and gas composition.

[189]

To study the effect of air preheating on fuels combustion on a

grate by pot fixed-bed experiments and real-scale MSW plant

observations.

(1) Preheating of PA acts as a catalyst for the ignition on a grate rather than

only drying of the waste; (2) pot furnace experiments have only a limited

value in studying grate boiler combustion.

[190]

To study numerically the effects of fuel properties (i.e., LHV,

density, particle size, and packed bed porosity) on biomass

combustion characteristics on a grate.

(1) Average burning rate is mostly affected by fuel particle size. (2)

Combustion stoichiometry is equally affected by LHV and particle size. (3)

Density has the strongest effect on solid temperature. (4) LHV and particle

size have the strongest effect on CO; LHV and density have the dominant

effect on CH4; and particle size has the greatest effect on H 2 concentration at

the bed top.

[78]

To investigate the importance of particle size and density of fuel

on biomass conversion in a packed bed by a 1D model andtests.

(1) Particle density has a very small effect on the conversion rate in a packed

bed. (2) Particle size has a significant influence on the conversion of a packedbed. In a bed of large particles (30mm 30mm 30 mm), a clear

temperature difference exists between gas and solid. In a bed of small

particles (3 mm 3 mm 3 mm), the temperature difference is small and

could be neglected in modelling.

[55]

To study the conversion of biomass on the grate of a 25 MWe

forward reciprocating grate boiler by modelling and

experiments.

(1) By varying grate speed to obtain constant bed height, the furnace can

achieve 499% of conversion efficiency with 40–50% of normal PA supply. (2)

Char conversion is much slower than devolatilization.

[65]

A sensitivity analysis of the uncertainty of model parameters

related to heat and mass transport, reaction rates, and

composition of volatiles.

(1) Prediction of ignition rate and temperature peak is relatively insensitive

to the uncertainty in the parameters; (2) composition of volatiles affects

greatly gas concentration in the bed and gas ignition.

[191]

Group 2—CFD modelling of mixing and combustion in the freeboard

To study the effect of advanced secondary/over-fire air Ecotube

system in a 15 WMth biomass waste-fired grate boiler using

CFD.

New Ecotube air system generates a considerable improvement in efficiency

for biomass combustion in the grate boiler.

[80]

To model and validate a 50 MWth wood chips-fired grate boiler,

and then to predict the effect of an ‘ECO’ air system on NO x

emissions.

With an improved SA supply (by ‘ECO’-tubes), 30% NO x reduction can be

achieved.

[57]

To evaluate the effect of an ‘Ecotube’ air supply system on

combustion and emission from a 25 MWth biomass-fired

grate boiler using CFD.

The new SA system can result in a higher furnace flame occupation

coefficient, a more uniform heat release, a longer life of combustion

chamber, a lower level of pollutant emissions, and combustion noise.

[39]

To investigate the effect of air staging and flue gas recirculation

on flue gas burnout, mixing, and temperature distribution

using CFD.

Appropriate air staging and flue gas recirculation have a considerable

potential to optimize the mixing and improve the temperature distribution

and control to prevent slagging in biomass grate boilers.

[192]

Combined simulation of a 1 MWth wood chips-fired sloping grate

hot-water boiler by interactively employing a 1D bed model

and CFD.

(1) The predicted results of the fuel bed model are rather insensitive to the

freeboard conditions and more likely to be affected by fuel bed properties.

(2) Minor changes in SA largely reduce CO emission.

[56]

To study a 12T/h MSW-fired grate boiler by FLIC/fluent combined

simulation, as well as measurements.

(1) MSW on the grate is ignited at 1.8–2.0 m away from the waste entrance;

the measured maximum bed temperature is 1000–11281C with big

fluctuations up to 800 1C. (2) Improving SA is necessary to reduce particle

carry-over to boiler tubes and to increase the heat transfer.

[48]

To evaluate MSW combustion process in a pilot grate furnace

using CFD and experiments, under centre-flow operation.

(1) CFD and experiments show good agreement on flue gas burnout in

combustion chamber. (2) CFD is a valuable tool for further study on the

effect of PA supply, fuel mass flow, and grate movement.

[49]

To present a CFD analysis of a 33 MWfuel straw-fired grate boiler. (1) Model predictions show a good agreement with available measurements(temperature and species). (2) Poor mixing between bulk flow and SA jets is

partly responsible for high CO and UBC in fly ash.

[193]




ARTICLE IN PRESS

Table 9 (continued )


CFD study of the dynamics of particles (spatial distribution,

residence time, momentum at outlet) in a 40 MW wood

chips-fired grate furnace.

By changing the air supply configuration (to form stronger recirculation

zones and higher level turbulence), the residence time of particles could be

prolonged.

[61]

Combined simulation of a 150 tons/day waste-fired grate boiler. (1) A strategy of combining CFD of gas flow field with waste bed combustion

is presented and tested. (2) More realistic submodels of waste bed

combustion would be helpful in this method.

[50]

To model and validate biomass combustion in a semi-industrial

grate boiler (firing 0.2 T/h biomass) and a full-scale grate

boiler (firing 13T/h biomass), and then to optimize them

through modelling.

(1) By modifying the heat release profile over the grate and the split

assumption for C and H in the calculation of the species released from the

fuel bed, the waste incineration model is very capable of predicting biomass

combustion in a grate boiler. (2) Boiler performance can be improved by

optimizing the ratio of PA/SA and the ratio of front wall SA to rear wall SA.

[194]

To model and optimize a 25 MWe MSW-fired reciprocating-grate

boiler using CFD coupled with fuel bed model (FLIC).

In the design case, there exists a large flow recirculation zone in the

radiation pass, which is not good for mixing and combustion, and can be

avoided by changing the distribution of SA jets.

[67]

Using CFD as a tool to upgrade grate-firing systems for improved

boiler operation and reduced emissions.

(1) Multiple rows of small OFA ports result in poor mixing with channelling

of gas up the boiler centre. (2) Poor combustion is overcome by upgrading

OFA system, a consistent conclusion from 30 boilers.

[181]

To investigate by using CFD the important parameters relevant

for design of a multi-fuel low-NO x grate furnace and to derive

guidelines for design of grate furnaces of 0.5–10 MWth

capacity.

(1) The designs of flue gas recirculation nozzles, SA nozzles, air staging and

combustion chamber geometry are investigated. (2) Injection of recycled

flue gas above the fuel bed is good for lower CO and NOx. (3) SA staging is

good for CO reduction. (4) The size and location of recycled flue gas nozzles

and SA nozzles and the jet speeds are important for mixing, combustion and

emissions.

[195]

To retrofit a 1975 vintage travelling grate stoker boiler in a paper

mill in Louisiana, USA, using CFD.

Simulations suggested the new air system and fabric stoker seal be installed.

The boiler was finally retrofitted on the basis of CFD indications.

[63]

To study how to improve air/gas mixing in a waste incinerator to

reduce incomplete combustion and lower emissions.

SA nozzle configuration is important: (1) mixing can be improved by

selecting larger inter-jet spacing, stronger jet speed; (2) staggered

arrangement of two opposite nozzle arrays is more effective.

[196]

To model a biomass grate boiler and locate the reasons for the

always over-predicted temperature in primary combustion

zone.

(1) Turbulence at grate inlet makes no difference to CFD results; (2) over-

predicted temperature is not due to the bed model; (3) mixture fraction/PDF

is better than eddy-dissipation but still over-predicts gas temperatures in

the primary combustion zone.

[197]

To introduce grate firing and also to present a CFD modelling of a

20MWe renewable energy, air-cooled travelling grate boiler.

(1) Care must be taken with SA systems to ensure radial and axial speeds of

SA jets match bulk gas velocity in furnace for proper mixing. (2) Balancing of

SA & PA is also important.

[3]

To establish a reliable baseline CFD model for a 108 MWfuel straw-

fired grate boiler for the purpose of optimizing design and

operation, using a thorough sensitivity analysis in CFD and a

2-day measuring campaign on the boiler.

(1) Raw input data and mesh are important in modelling of grate boilers,

even more important than the bed model in terms of overall CFD results. (2)

Mainly due to the raw inputs (e.g., the conditions of the walls and air-jets

under irregular deposits, the non-continuous biomass feeding and grate

movement and the combustion instabilities in the fuel bed), it is not thateasy to establish a reliable baseline model. (3) Staggered OFA jets are

favourable for forming double rotating flow in horizontal cross-sections in

burnout zone. However, the jets momentum and spacing could be further

optimized.

[53]

Group 3—Modelling of NO x formation and emissions from biomass-fired grate boilers

To develop a model for NO x emissions from biomass grate

furnaces.

(1) A new flamelet combustion model is developed. (2) The preliminary

results by applying the model to a 2D biomass-fired grate boiler are

encouraging.

[198]

To improve the understanding of combustion and NO emission

characteristics by studying numerically and experimentally

the related processes in an 8–11kW updraft wood pellets-

fired furnace.

(1) Biomass combustion in grate boiler can be effectively controlled by SA

and OFA jets. (2) The burnout zone is not sensitive to the bed combustion

process due to the high-speed flow from SA and OFA jets. (3) The rather high

flame temperature (1800 K) in this furnace leads to high NO emission from

thermal-NO mechanism. (4) In air-rich burnout zone, N2O-intermediate

mechanism dominates.

[107]

Combined simulation of a 38 MWe straw-fired grate boiler to

study its performance and the effect of variation in operation

conditions.

(1) Most of the NO is formed in the downstream combustion chamber. (2)

Fuel moisture content is limited to below 25% to prevent excessive CO

emission without compromising the plant performance.

[54]

To model SNCR application to a full-scale MSW-fired grate boiler. (1) The model is successfully evaluated against operational data

(temperature, NO and gas velocity). (2) The appropriate injection port of

reduction material for maximum NO reduction could be determined.

[62]

To propose a thermochemical model for the simulation of the flue

gas cleaning system of an RDF incineration plant.

The model simulates operation of flue-gas treatment section (NO x reduction,

SO2 and HCl scrubbing) and combustion section (grate incinerator, post-

combustion chamber) by using a simplified approach. The simulation results

are validated with operating data, indicating the model can be a practical

tool.

[51]

Group 4—Modelling of deposit formation in biomass-fired grate boilers

To examine possible practical influences on aerosol formation in

biomass-fired boilers and to determine the amount and

chemical composition of particle emissions.

A plug flow model is developed, considering gas-phase modelling by means

of thermodynamic equilibrium calculations and a kinetic approach for

modelling gaseous sulphate formation. The main particle formation and

precipitation mechanisms are considered.

[135,139]

To model the gaseous sulphation of alkali hydroxide and alkali

chloride since the formation of gaseous alkali sulphates may

yield aerosols and also contribute to deposition and

corrosion.

(1) A reaction mechanism for sulphation of alkali metals is proposed. (2)

Modelling predictions are not sensitive to the estimated properties in the

alkali subset. (3) The predicted degree of sulphation is affected mainly by

the rate of SO2 oxidation and the production of chain carrier in the system.

[154]




combustion in industrial grate boilers, provided that the correct

amount of mass, elements, and heat is released from the fuel bed

into the freeboard.

Approach III . Recently, separate bed models have been devel-

oped to study biomass conversion in the fuel bed on the grate, for

example, in the Sheffield University Waste Incineration Centre

(SUWIC) chaired by Prof. Swithenbank and the group headed by

Prof. Choi at the Korea Advanced Institute of Science and

Technology (KAIST). The ignition front and the combustion front

in fuel beds are tracked, and the temperatures, species, and

velocity at the fuel bed top are also solved, which are used as inlet

conditions for the freeboard modelling, see, for example,

[48,50,54,56,67,193]. The sensitivities of biomass properties (e.g.,

moisture content, particle size and density, solid conductivity,

heating value) and process parameters (e.g., heat and mass

transfer rates, bed porosity, devolatilization rate, primary air flow

rate, heat capacity of both the gas and solid phases) on the

conversion rate, temperature, and gas compositions are also

studied by using separate bed models [55,76–79,191].

Basically, approach III is to solve mass, momentum, energy, and

species balance equations for gas and solid phases, with necessary

process rate equations and empirical correlations/sub-models

used for the closure of the balance equations. However, one may

find huge inconsistencies in the sets of equations solved in the

different works, not only in forms (e.g., in divergence operators, inEinstein convention, or in algebraic forms) but also in some of the

terms (e.g., some terms may be solved in one application but

neglected in the other) or even in the inclusion of some transport

equations (e.g., the inclusion of momentum equations or not). The

large diversity in the sets of equations could be understood due to

the complexity of the fuel bed on the grate or due to the specific

subjects under different studies. For instance, quite some model-

ling efforts were done for fixed-bed combustion of biomass, in

which one-dimensional unsteady heterogeneous models were

solved for the fixed bed and then the time elapsed since ignition in

the fixed bed was mapped to the horizontal distance away from

the start point on the travelling grate in industrial grate boilers.

This kind of approximation may be acceptable for travelling grate

combustion as a result of the relatively small horizontal gradientsin temperatures and species concentration in some industrial

grate boilers. Some efforts were done directly for travelling grate,

in which 2D bed models were developed.

To have a better overview of the governing equations and to

develop a more general computer code for biomass conversion in

the fuel bed on the grate, MFIX (Multiphase Flow with Interphase

eX changes) [204] could be one of the most useful sources. MFIX is

a general-purpose computer code developed at the National

Energy Technology Laboratory (NETL) for describing the hydro-

dynamics, heat transfer, and chemical reactions in fluid–solid

systems. MFIX code is based on a generally accepted set of

multiphase flow equations, which are summarized in [205], and

the source code is available through its website, http://

www.mfix.org [204]. MFIX calculations give transient data on

the three-dimensional distribution of pressure, velocity, tempera-

ture, and species mass fractions. Though MFIX is mainly used for

describing BFBs and CFBs and spouted beds, which are different

from the fuel bed on a grate, the basic governing equations and

the programming techniques are still the same and useful for the

development of bed models for grate-fired boilers.

The biomass bed on the grate can be viewed as a reacting

gas–solid system, which includes one gas phase and one solid

phase. The gas phase has 6 or more different species, e.g., O2, N2,

CO, CO2, CmHn, and H2O vapour. The solid phase has 4

components, i.e., moisture, volatile matter, fixed carbon, and inert

ash. For such an air–solid system, the general governing equationsfor modelling can be summarized as follows.

Continuity equations: The continuity equation for the gas phase

is

qðfrgÞ

qt þ r ðfrgugÞ ¼ S g ðfor gas phaseÞ (10)

where f, rg, ug and S g represent the volume fraction of the gas

phase (i.e., void fraction in fuel bed), the material density of the

gas phase [kg/m3], the gas-phase velocity vector [m/s], and the

conversion rate from solid to gases due to evaporation, devola-

tilization, and heterogeneous reactions [kg/(m3 s)], respectively.

The process rate equations for evaporation, devolatilization, and

char oxidation can be found in the relevant literature (see Table 9)or in MFIX.

ARTICLE IN PRESS

Table 9 (continued )


To develop a model for predicting the formation of ash deposits in

biomass-fired plants and implement it in CFD.

(1) It covers the release of fly ash particles and ash-forming vapours from

fuel bed, their transport and deposition on boiler walls. (2) The application

in a 440kWth boiler shows plausible results.

[199]

To develop a model for predicting the formation of ash deposits in

biomass-fired plants and demonstrate it in an industrial grate

boiler.

A deposition model is established and demonstrated, in which the deposits

are built up from fly ash particles (2–250mm) by inertial, turbulent, and

thermophoretic mechanisms and KCl vapours-formed particles (0.5 mm) by

diffusion, turbulent, and thermophoretic mechanisms.

[146]

To develop and validate a dynamic model of ash deposit growth

and shedding on a horizontal probe in a straw-fired grate

boiler.

(1) The model covers deposit growth, and also shedding by deposit surface

melting. (2) Model predictions agree with the measured evolutions of the

deposit weight, heat uptake and deposit shape. (3) KCl condensation

initiates the deposit formation and fly ash particle inertial impaction is the

main contribution to the deposit growth. (4) The deposit growth rate is

balanced by the shedding rate after 285 h in this study.

[69]

Group 5—Modelling or assessing of the discontinuous effects

To assess the effect of grate movement and waste feeding cycles

in full-scale MSW incinerator by modelling and tests.

Transient process is observed due to MSW feeding cycles and grate

moving–rest cycles. In flue gas, O2 in flue gas varies in 6–12%, temperature in

850–10101C. Combustion in fuel bed is also dominated by big fluctuations in

temperature and O2: about 600 1C and 12% from the spikes to dips,

respectively.

[47]

To model the discontinuous incineration process in reciprocating

grate boilers for control purposes with the aim of reducing

oscillations through a model-based control system.

Discontinuous features (e.g., discontinuous feeding of MSW, discontinuous

movement of the burning waste by reciprocating grates) are mainly

responsible for the high oscillations of process variables. The modeldeveloped show good agreement with experiments and is used for the

specified control purposes.

[200]


http://www.mfix.org/






The continuity equation for the solid phase can be expressed as

qðrsÞ

qt þ r ðrsusÞ ¼ S g ðfor solid phaseÞ (11)

where e, rs, and us are the volume fraction of the solid phase,

e ¼ 1f, the material density of the solid phase [kg/m3], and the

solid-phase velocity vector [m/s], respectively.

Momentum equations: In principle, momentum equations

represent, ‘‘mass times acceleration per volume equals to the

sum of all the external forces per volume’’. Therefore, all the

terms should have the unit N/m3. If neglecting the inter-

phase momentum transfer due to the inter-phase mass

transfer (e.g., heterogeneous reactions), which is accounted

for in MFIX, the momentum equation for the gas phase can be

expressed as

qðfrgugÞ=qt þ r ðfrgugugÞ |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Densityacceleration

¼ fr pg |fflfflfflffl{zfflfflfflffl} Pressure

force

þ r ðfsÞ |fflfflfflffl{zfflfflfflffl} Viscous

force

þ frgg

|ffl ffl{zfflffl} Gravity

bgsðug usÞ

|fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} Momentum

transfer due to

interphase forces

þ f g |{z} Resistance

due to poroussurfaces

(12)

where pg and s represent the pressure in gas phase [Pa] and the

viscous stress tensor for gas phase [Pa], respectively. bgs is the

coefficient for the inter-phase forces, mainly the drag force in

most cases, [kg/(m3 s)]. Different correlations can be found in

MFIX for the calculation of the gas–solid momentum inter-phase

exchange. f g is the gas flow resistance due to the porous surfaces

[N/m3], which is usually calculated by a porous media model, for

example as is done in MFIX.

The momentum equation for the solid phase can be written as

qðrsusÞ=qt þ r ðrsususÞ |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Densityacceleration

¼ r pg |fflfflffl{zfflfflffl} Pressure

force

þ r ðssÞ |fflfflfflffl{zfflfflfflffl} Solid

stresses

þ rgg |ffl{zffl} Gravity

þbgsðug usÞ |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} Momentum

transfer due tointerphase forces

þ f g |{z} Momentum

gain from gratemovement

(13)

where ss and f grate represent the solid-phase stress (or granular

stress) tensor [Pa], and the momentum transfer from the

mechanical movement of grate (e.g., travelling, vibrating, or

reciprocating movement) [N/m3], respectively. In MFIX, the

granular stress equations are calculated on the basis of kinetic

theory and frictional flow theory, and the resulting constitutive

equations can be seen in detail in [205]. Since the constitutiverelations contain granular temperature, a separate transport

equation for granular temperature or an algebraic granular energy

equation is used in MFIX to solve the granular temperature.

Actually, the momentum equation for the solid phase has not

really been solved in any modelling effort for biomass conversion

on the grate, probably due to the difficulty with the solid-phase

stress tensor, or probably due to insignificant movement of the

solids in a fixed bed, or the assumption hereof. In some modelling

efforts which are directly devoted to travelling grates, the

horizontal solid-phase velocity is pre-defined, whilst the vertical

component of the particle velocity in the fuel bed is calculated

from the solid-phase continuity equation [65,189].

Species transport equations: The transport equation for then-th species (e.g., O2, N2, CO, CO2, CmHn, and H2O vapour) in the

gas phase is

qðfrgY g;nÞ

qt þ r ðfrgugY g;nÞ

¼ r ðfrgDg;nr Y g;nÞ þ Rg;n ðfor gas phaseÞ (14)

where Y g,n, Dg,n and Rg,n represent the mass fraction of the n-th gas

species, the diffusion coefficient of the n-th gas species [m2/s], and

the rate of production of the n-th gas species due to evaporation,devolatilization, and combustion [kg/(m3 s)], respectively.

The conversion equation for the i-th solid-phase components

(i.e., moisture, volatiles, fixed carbon, ash) can be expressed as

qðrsY s;iÞ

qt þ r ðrsusY s;iÞ ¼ r ðrsDsr Y s;iÞ

þ Rs;i ðfor solid phaseÞ (15)

where Y s,i, Ds and Rs,i are the mass fractions of i-th particle

compositions, the diffusion coefficient of the solid phase [m2/s],

and the rate of conversion of the i-th solid species due to

evaporation, devolatilization, and heterogeneous combustion

[kg/(m3 s)].

There are some uncertainties with the diffusion coefficients, in

particular, the solid-phase diffusion coefficient, Ds. The movementof a grate enhances the solid mixing in the fuel bed on the grate

and may have important influence on biomass conversion on the

grate. Therefore, the solid-phase diffusion on different types of

grates has been studied experimentally and correlations for the

solid-phase diffusion coefficients have been proposed, see

[186,206].

Energy equations: The energy equation for the gas phase can be

expressed as

frgC pgqT gqt

þ ug r T g

¼ r ðlgr T gÞ

þ hgsðT s T gÞ

þ DH g þ _Q rad;g (16)

where C pg, T g, lg, hgs, DH g and _Q rad;g represent the specific heat of

the gas phase [J/(kg K)], gas-phase temperature [K], gas-phase

conductivity [W/(m K)], gas–solid heat transfer coefficient cor-

rected for inter-phase mass transfer phase [W/(m3 K)], heat of

reaction in gas phase [W/m3], and radiation heat source to the gas

phase [W/m3], respectively. The calculations of these terms are

relatively simple though different correlations may exist and be

used in different efforts. One of the arguments could be on the

role of the radiation heat source. The radiation certainly plays an

important role in initializing the ignition flame on the top of the

fuel bed. However, the heat of reaction could dominate over

radiation in the propagation of the flame in the fuel bed.

Similarly, the energy equation for the solid phase can bewritten as

rsC psqT sqt

þ us r T s

¼ r ðlsr T sÞ

hgsðT s T gÞ

þ DH s þ _Q rad;s (17)

where C ps, T s, ls, DH s and _Q rad;s represent the specific heat of the

solid phase [J/(kg K)], solid-phase temperature [K], solid-phase

conductivity [W/(m K)], heat of reaction in solid phase (including

the heat loss due to moisture evaporation and heat generation due

to char oxidation) [W/m3], and radiation heat source to the solid

phase [W/m3], respectively. In this modelling framework, the

isothermal condition (i.e., Biot number less than 0.1) is assumed

for biomass particles. However, some tests show that thetemperature gradients may not be neglected for big particles, for

ARTICLE IN PRESS




instance, thermally thick biomass fuels over 35 mm can develop a

temperature gradient over 400 1C in the particles at the flame

front under ordinary combustion conditions [207].

All the equations can be expressed in the standard form solved

in finite volume method,

qðrFÞ

qt |fflffl{zfflffl} Transient

þ r ðrFuÞ |fflfflfflfflfflffl{zfflfflfflfflfflffl} Convective

¼ r ðGr FÞ |fflfflfflfflfflffl{zfflfflfflfflfflffl} Diffusion

þ S F |{z} Source

(18)

With basic programming skills and using the finite volume

method, the modelling of biomass conversion on the grate may be

done. It would be better to programme on the basis of the

complete equations, in which the uncertain process parameters

(e.g., the inter-phase heat transfer coefficient) may be calculated

by separate subroutines or functions. 1D or 3D, and inclusion of

part of the contributions or all the contributions in the source

terms do not really add big difficulties to the programming. With a

general and structured computer code, it would be much easier

and more reliable to investigate the effects of some sub-models or

correlations (i.e., the sensitivity analysis) by simply replacing the

corresponding subroutine or function. Comparatively, there are

more uncertainties with the process parameters in the fuel bed ingrate-fired boilers, due to, for example, the relatively poor mixing

and the channel formation [208] and the very common combus-

tion instabilities [64] in the fuel bed on the grate. This also makes

it necessary to conduct the sensitivity analysis on the basis of a

reliable set of equations and a general computer code for the

modelling of biomass conversion in the fuel bed on the grate.

As an example, the second and the third approach are

demonstrated for the straw-fired water-cooled grate boiler shown

in Fig. 1(b) to show the differences. Fig. 13 shows the experience-

based, straw conversion rates along the grate and the primary air

distribution measured. For instance, biomass evaporation rates

85%, 15%, 0% and 0% along the grate length mean that in this boiler

85% of the moisture in the biomass is released in the first 14.6%

of the grate length, the remaining 15% of the moisture is released

in the second 20% of the grate length, and no moisture is released

in the last two grate sections (33.1% and 32.3% in length,

respectively). Based on the conversion rates and the primaryair distribution, heat and elements balance are used to derive

the species, velocity, and temperature of the combustion gas at

the bed top: the results are shown in Fig. 14. Fig. 15 shows the

counterpart calculated by the third approach based on the same

data of straw and primary air. Some differences are observed from

Figs. 14 and 15. However, they should result in the same amount

of elements, mass, and heat flow into the freeboard. Please be

aware that the gas velocities, shown in Figs. 14 and 15, are not

the velocities of the primary air. The superficial velocity of the

primary air at the bottom of the fuel bed is much lower, in the

range of 0.21–0.65 m/s in this grate-fired boiler.

Besides the models for the biomass conversion on the grate,

some modelling work has also been done to investigate the effects

of grate components themselves or to characterize fuel particlesmixing and residence time on the grate, as can be seen in Table 9.

4.3.2. CFD modelling of the mixing and combustion in the freeboard

Compared to pollutant formation, deposition and corrosion,

and biomass conversion in the fuel bed on the grate, the

combustion in the freeboard may be more related to combustion

physics. When the gases are released from the fuel bed, they mix

with the secondary air and combust in the freeboard. The gaseous

combustion is generally fast compared to the rate of mixing.

Therefore, fluid mechanics (i.e., mixing) plays a very important

role in the combustion and pollutant formation in the freeboard,

particularly for grate-fired boilers which have relatively low

combustion temperatures.

The majority of the existing CFD modelling of grate-firedboilers focuses on the mixing and the optimization, as can be seen

in Table 9. The poor mixing in grate-fired boilers could be in the

form of, for example, insufficient mixing between the bulk flow

and the SA jets, or insufficient occupation of the flue gas or flame

in the volume of the freeboard, or the formation of channelling

flow in the freeboard. The CFD modelling results show that the

mixing in the freeboard can be improved by, for example,

advanced air supply systems, optimized secondary air jets

(momentum, configuration, location, and spacing), or adjusted

split between secondary air and primary air (see Table 9 for

details). The basic ideas are to improve the momentum or

penetration of SA jets, form local recirculation zones, or form

rotating flows on horizontal cross-sections in the freeboard. The

mixing can also be improved through grate systems, for instance,

using good fabric stoker seal for low excess air or designing new

grates for improved distribution of primary air.

ARTICLE IN PRESS

Fig. 13. The experience-based conversion rate as a function of position on the

grate.

Fig. 14. Approach II: the gas species (left), gas temperature, and velocity (right) at the fuel bed top.




4.3.3. Modelling of NO x formation and emissions from grate-fired

boilers burning biomass

The major source of NO x from biomass-fired grate boilers is

fuel NO x. Most of the existing understanding of fuel nitrogen

conversion in solid-fuel-fired systems involves coal as a fuel.

Comparatively little is known about other fuels such as biomass,

which limits the modelling capability of NO x formation and

emissions from grate-fired boilers burning biomass. The more

crucial point may be the release of the NO x precursors from the

fuel bed on the grate under different environments for different

biomass fuels. As shown in Table 9, there are some, but still

limited, efforts in the modelling of both NO x precursors released

from the fuel bed and NO x formation in the freeboard. For the CFD

modelling of NO x formation in the freeboard, the NO x precursors

released from the fuel bed on the grate, which are used as the

grate inlet boundary conditions, are mainly based on assumptions

(as seen in Table 6). This could be improved for a better modelling

of NO x emissions from grate-fired boilers burning biomass.

4.3.4. Modelling of deposit formation in grate-fired boilers

burning biomass

The modelling of deposit formation in grate-fired boilersburning biomass is very complicated, as a result of the complex

mechanism of deposit growth and shedding. Alkali vapours, fly

ash particles, and aerosols all contribute to the deposit formation.

Modelling of deposit formation involves the release of the

precursors of formation of alkali vapours and particulate matter

from the fuel bed, the reaction and transport of the vapours and

particulate matter in the freeboard, the transport of the vapour

and particulate matter to the furnace walls, and their sticking

propensities. The existing efforts show an encouraging potential

in estimating the deposit formation in grate-fired boilers burning

biomass; however, these efforts are still quite preliminary.

4.3.5. Modelling or assessing of the discontinuous effects

Quite often, grate-firing of biomass is characterized by somediscontinuous effects, e.g., the discontinuous biomass feeding and

the discontinuous grate movement. These discontinuous pro-

cesses certainly affect the plant operation and control, e.g., big

fluctuations or oscillations in process variables. However, very

little work has been done on this aspect.

5. Future R&D

Based on the knowledge and achievements already acquired,

more efforts in biomass combustion are still needed to resolve the

existing problems, particularly when we have to face the

increasing price of fossil fuels and the more stringent target to

utilize renewables. Further R&D related to grate-firing of biomassmay be suggested as follows.

5.1. Mechanism study of combustion chemistry for grate-firing

of biomass

Combustion chemistry and combustion physics are the two

foundations of any combustion technology. Combustion physics,

for instance, how to improve the mixing in the combustion

chamber, how to develop or improve the physical models to

extend their prediction capabilities, and how to make use of

experimental facilities or techniques (e.g., electric probes, optics,

acoustics, spectroscopy and pyrometry) to measure or monitor the

combustion processes, may be more general for combustion

processes. For grate-fired boilers burning biomass, some of these

have been extensively covered by current efforts, such as, mixing;

some of these are in a great need of enhancement, such as,

comprehensive experimentation; some of these may be less

pertaining to grate-fired boilers burning biomass, for instance,

improvement of some physical models including suitable sub-grid

models. Combustion physics has not been explicitly highlighted as

a separate issue throughout this paper.

Comparatively, combustion chemistry is more dependent on

the fuel and the combustion technology. Combustion of biomass

in grate-fired boilers has substantially different conversion

characteristics than those in other combustion technologies(FBC or suspension combustion). So far, the majority of the

mechanism studies of combustion chemistry in grate-fired boilers

burning biomass go to the combustion characteristics, e.g.,

ignition, devolatilization, char oxidation, reactivity of the released

volatiles, and char, as well as fuel nitrogen conversion. Some

efforts have also been made in the transformation of inorganic

elements, e.g., K, Cl, and S, in grate-firing conditions, in order to

facilitate the study on the ash formation, deposit formation,

and Cl-induced corrosion. The speciation and transformation of

heavy metals during grate-firing of biomass have also been

involved to a small extent, since they significantly affect the

formation of some toxic pollutants (e.g., PCDD/PCDF and heavy

metals emissions) and the utilization and disposal of ash. These

existing mechanism studies are still far from being sufficient andmay have to be strengthened by different extents. Without

sufficient and detailed knowledge of the combustion chemistry,

it is less likely to build up a reliable CFD model, to correctly

optimize/design the combustion system, and to propose efficient

measures to control pollutant emissions. For instance, a greater

part of heavy metals and chlorinated organic compounds (e.g.,

PCDD/PCDF) is bound in the fly ash during the combustion of

some biomass fuels, e.g., MSW. In order to propose efficient

measures to control the concentration of the toxic components in

the fly ash and to dispose of the fly ash, the transformation and

conversion characteristics of all the relevant elements (e.g.,

carbon, heavy metals and Cl) during biomass combustion, as well

as more detailed gas-phase chemistry and how it participates in

the heterogeneous chemistry, should be studied and well under-stood.

ARTICLE IN PRESS

Fig. 15. Approach III: the gas species (left), gas temperature, and velocity (right) at the fuel bed top, calculated by the model [76].




To meet the increasingly stringent target to utilize the

renewables, more and more new biomass fuels will be exploited

and fed into grate-firing systems for heat and power production.

This also demands a significant amount of mechanism studies of

their combustion chemistry, in order to fire them efficiently and

cleanly.

The effects of different additives used in biomass-fired grate

boilers are also an important aspect in the mechanism study of combustion chemistry which needs improvement. Additives could

be used in grate-firing of biomass to achieve different purposes,

for instance, to improve combustion or to mitigate some problems

(e.g., emissions, deposition, and corrosion).

5.2. Advanced monitoring, testing, and experimentation

Some details of the grate-firing systems are not readily

accessible. Older boilers are particularly troublesome since they

even lack good flow measurement devices and monitoring

equipment. As discussed in [53], the uncertainties with the

details in biomass-fired grate boilers challenge the modelling,

operation, and optimization, for instance:

In large-scale grate-firing systems, there is often more than one

grate between the two side-walls at the bottom of the furnace.

Quite normally, the feeds onto the different grates are not

identical. Moreover, the feeding of biomass is not necessarily in

a continuous manner. The biomass feeding cycles, together

with the grate moving–rest cycles, may cause significant

fluctuations in the combustion process in the grate boilers [47].

The combustion instabilities in the fuel bed on the grate, e.g.,

local burnouts, channelling formation, and spatially uneven

fuel-bed thickness [64].

The uncertainties with the process parameters and physical

properties of biomass in the fuel bed on the grate, e.g., the

mixing rate, the heat transfer and mass transfer rates, the size

and shape distribution of biomass particles, the porosity of thefuel bed, and so on.

The uncertainties with wall conditions. The deposit formed on

the furnace walls as well as on heat-exchangers makes it

difficult to estimate the right wall conditions.

The uncertainties with air distribution and air-jet conditions. Air

jets play a very important role in the mixing in the freeboard. Quite

often, in large-scale grate-fired boilers, there are a few different

groups of air nozzles, and amongst each group there are several or

even many individual nozzles. Normally, only the total air flowrate

to each group or even to a few groups is monitored in the plant.

The air flow through different individual nozzles in the same

group may be unevenly distributed and in some cases big

deviations could exist. The deposit formed on the air nozzles

may give rise to an even bigger uncertainty with the conditions of the air jets: the irregular deposit on the air nozzles not only

deflects the direction of the air-jets but also re-distributes the air

flowrates through individual nozzles [53].

Without correct inputs, it is not possible to generate a reliable

model or CFD representation, from which suggestions/measures

on how to guide/improve the operation/design are often derived.

So, advanced monitoring, tests, and experimentation are needed

in order to obtain the necessary raw data as reliably as possible.

5.3. General and comprehensive model for biomass conversion in the

fuel bed

Grate and biomass combustion in the fuel bed on the grate arethe most specific topics in grate-fired boilers burning biomass.

Biomass combustion on the grate determines not only the main

combustibles but also the formation precursors (e.g., of particu-

late matter, pollutants, deposition, and corrosion) released into

the freeboard. So, grate and biomass combustion on the grate play

an important role in the overall performance of a grate-fired

boiler.

As discussed earlier, some efforts have been successfully made

to develop models for the biomass conversion in the fuel bed onthe grate, mainly to study the ignition and combustion character-

istics of biomass in the fuel bed. However, the models need to be

generalized, on the basis of a well-accepted set of multiphase flow

equations and without introducing too many assumptions or

simplifications at the very beginning stage of the development of

the model and the code. The models need also to be extended to

more topics, e.g., fuel nitrogen, fuel inorganic elements, and heavy

metals, to study the conversion and release characteristics of the

relevant species in the fuel bed, as well as to provide reliable

precursors for freeboard modelling. These demand substantial

knowledge in combustion chemistry of biomass conversion under

grate-firing conditions.

The models should satisfy some basic requirements. One of the

necessary conditions for the models is that the combustion gasmust carry the correct amount of mass, heat, and elements into

the freeboard. Knowing the data of the biomass fuel, primary air,

and external heat flux incident onto the fuel bed, the models will

output the profiles of species concentration, temperature, and

velocity of the combustion gas, leaving the bed top into the

freeboard (see Fig. 14 or 15 for examples). An integral analysis

must be carried out for the profiles of the species, temperature,

and velocity at the top of the fuel bed, to ensure the model itself

conserves the elements, mass, and heat.

5.4. Advanced CFD modelling

A reliable baseline CFD model is vital in CFD analysis, for

instance, for the diagnosis and optimization of the existing grate-firing plants, or for the design of new grate-firing systems.

Different from modelling of suspension-firing systems, CFD

modelling of grate-fired boilers may have a separate bed model

for biomass conversion in the fuel bed, as well as large gradients

in species concentrations at the bed top. The reliability of a CFD

model depends heavily on the quality of the raw input data, the

mesh, the models (including the separate bed model), and the

numerical methods. However, these aspects may not be consid-

ered equally in some of the existing CFD work. It is less likely to

generate a reasonable CFD representation for grate-firing systems

if over-highlighting one factor while neglecting others. As shown

in [53], in modelling of biomass-fired grate boilers, the main

effects of the models for biomass conversion in the fuel bed may

only be restricted to the vicinity of the fuel bed. In terms of theoverall flow and combustion patterns in the freeboard, the mesh

could play an even more important role. In CFD modelling of

grate-fired boilers, the raw input data could also be a challenge,

which demands advanced monitoring, tests, and experimentation

for the fuel properties in the fuel bed and the process parameters

both in the fuel bed and in the freeboard.

Advanced CFD modelling of grate-fired boilers should also be

extended from the mixing and combustion in the freeboard to

other important topics. For instance:

Modelling of pollutant emissions (e.g., NO x, PCDD/PCDF, heavy

metal, size and compositions of fly ash) will be helpful to

develop efficient emission control measures. However, it needs

sophisticated fundamental knowledge, for example, on theconversion of biomass fuel nitrogen, inorganic elements, and

ARTICLE IN PRESS




heavy metals, more detailed gas phase chemistry, and how it

participates in the heterogeneous chemistry.

Modelling of deposit formation has gained some concerns. In

biomass-fired grate boilers, the fly ash particles may dominate

in the total mass of the deposit on the tubes due to inertial

impaction mechanism. Biomass particles fired in grate boilers

are quite big and irregular (see [209] for reference). A reliable

estimate of the particles entrained into the freeboard (includ-ing size, composition, and flow rate), the trajectories and

heterogeneous reactions of the entrained particles in the

freeboard, the ‘‘stickness’’ of the particles when they are

transported to the wall surfaces, and the wall conditions all

play important roles in the prediction of deposit formation. Big

irregular particles of relatively low density have different fates

than small (point) heavy particles, which may demand

advanced tracking methodology [210]. Big irregular particles

are thermally thick, which could lead to different particle

conversion process due to the big internal temperature

gradients [207,211]. So, the modelling of deposit formation

may have to take these factors into account.

5.5. Optimization and modernization for better performance

Better performance includes higher combustion efficiency,

lower emissions (both gaseous and particulate pollutants), and

better reliability and availability.

Advanced combustion air system is highly required in modern

grate-fired boilers burning biomass. The multiple zones of

under-grate primary air can help to achieve an optimum

temperature distribution and good ash burnout. Advanced

secondary air supply, for instance, the air staging, staggered SA

jets on the opposite furnace walls, static mixing devices, and

tangentially arranged SA jets as discussed earlier, could be used

to optimize the mixing and combustion in the freeboard and

then to improve the efficiency, reduce emissions, as well as

mitigate the deposition and corrosion in the boiler.

Improved fuel-handling and feeding systems and advanced

combustion grates can enhance the gas–solid mixing on the

grate and reduce the excess air. For instance, the reciprocating

movements in a reciprocating grate or vibration frequencies/

amplitudes in a vibrating grate may be optimized to achieve a

good mixing in the fuel bed. Improved fabric seal of the grate

system (including stokers) will result in a low excess air ratio.

Pre-combustion or post-combustion measures may also be used

to lower pollutant emissions and mitigate the deposition or

corrosion problems, as discussed earlier.

6. Conclusions

Biomass combustion for heat and power production is

progressing relatively fast, not only in research but also in

commercialization. This review paper focuses on grate-firing of

biomass: the main R&D activities, the progress, and the problems,

all of which are primarily pertaining to grate-fired boilers burning

biomass. Fluidized bed combustion of biomass is also discussed to

some extent, mainly for comparison and for a better illustration of

grate-firing of biomass. Both the technologies show great

competence in biomass combustion because they have good fuel

flexibility and can fire a wide range of fuels of varying moisture

and ash contents. The differences between the two technologies

for biomass combustion are summarized.

Amongst the key elements of grate-fired boilers, the grateassembly and the advanced secondary air supply are highlighted.

The former represents the most specific component in grate-fired

boilers whilst the latter is one of the real breakthroughs in grate-

firing technology. The grate assemble plays an important role in

the gas–fuel mixing (and therefore biomass conversion) in the

fuel bed as well as the control of overall excess air in the boiler.

The key combustion mechanism (i.e., propagation of flame fronts)

in the fuel bed on the grate is also discussed, which not only

determines the release of heat and combustibles into thefreeboard but also affects the release characteristics of the

formation precursors of NO x, aerosol and ash particles, and

PCDD/PCDF. Advanced secondary air systems are widely used in

modern grate-fired boilers in order to enhance the mixing and

combustion in the freeboard, lower the pollutant emissions, and

mitigate other operational problems (e.g., deposition and corro-

sion). Advanced secondary air systems may include air-staging for

favourable combustion environment sequences and optimized SA

jets for enhanced mixing, for example, using staggered SA jets,

tangentially arranged SA jets, or static mixing devices to form

local recirculation zones or rotating flow or to increase the jet

penetration into the centre of the freeboard. Compared to air-

cooled grates, water-cooled grates are more flexible with the use

of advanced secondary air systems.Amongst the issues associated with grate-fired boilers burning

biomass, primary pollutant formation and control, deposit

formation and corrosion, modelling and simulations for diagnosis,

optimization, and new design are highlighted. Based on these, the

critical problems that may be addressed by further research and

development are outlined. Combustion chemistry and combustion

physics, the two foundations of all the issues or problems, are

discussed throughout the different issues or problems.

Primary pollutants formation and control: The pollutant emis-

sions due to incomplete combustion can be controlled by

improved combustion, in grate-fired boilers, which mainly

means by improved mixing in the freeboard as well as

increased residence time in the combustion zones. The

pollutant emissions from fuel properties (e.g., ash, heavy

metals, Cl, and S) can be reduced by pre-treatment of the

biomass, well-controlled combustion process, or post-combus-

tion systems. In order to develop efficient measures to control

the pollutant emissions, it is crucial to understand their

formation routines or mechanisms under grate-firing condi-

tions. More efforts need to be made in the fundamental

combustion chemistry to study the transformation, speciation,

conversion, and reaction of the relevant elements (e.g.,

inorganic elements and heavy metals) in biomass fuels under

grate-firing conditions.

Deposit formation and corrosion: In grate-fired boilers burning

biomass, the volatile alkali inorganic vapours and fine particles

may contribute to the initial deposit formation while the inertnon-volatile ash particles contribute to the build-up of the

deposit. The most severe corrosion is associated with the

deposits containing alkali chlorides mainly on the super-heater

tubes in the boiler. However, different opinions may exist on

the role of sulphation in the corrosion mechanism, as

discussed in the paper. Amongst all the measures to mitigate

or even eliminate the deposition and corrosion problems, the

use of additives gets most of the current concern, which can

raise the melting temperatures of the ash formed during

biomass combustion, or prevent the release of gaseous KCl or

react with KCl to form less corrosive components, or a

combination of these effects. Injection of additives into the

combustion gases in a certain temperature window upstream

of the superheaters, e.g., ChlorOut (ammonium sulphate), maybe more attractive since it is less dependent on the combustion

ARTICLE IN PRESS




technology. Due to the poor mixing and the reducing condition

in the fuel bed in grate-fired boilers, adding additives to the

fuel bed may be a less attractive option for grate-fired boilers,

at least compared to fluidized bed combustors. The funda-

mental chemistry on the transformation of the relevant

elements is required, if for instance the formation of the

inorganic vapours and fine particles and the relevant homo-

geneous and heterogeneous reactions is to be studied. Modelling and CFD simulations for diagnosis, optimization, and

new design: Modelling and simulation represent one of the

main efforts devoted to biomass combustion in grate-fired

boilers. However, a more general and comprehensive model for

biomass conversion in the fuel bed on the grate is yet to be

developed. Biomass conversion on the grate is the most

specific area and also plays a key role in the overall

performance of grate-fired boilers (e.g., efficiency, pollutant

emissions, deposition, and corrosion). For this purpose, a

generally accepted set of multiphase flow equations, which is

sufficient to describe the aerodynamics, heat and mass

transfer, and chemical reaction in the fuel bed on the grate,

is summarized. CFD modelling of the mixing and combustion

in the freeboard provides useful details, based on which someefforts of mixing optimization have been achieved. To develop

a reliable baseline CFD model for grate-fired boilers burning

biomass, the quality of the raw input data, the mesh, the

models (including the separate bed model for biomass

conversion in the fuel bed, as well as stand-alone sub-models

for particle tracking, particle conversion, ash deposition, etc.)

and the numerical methods all play important roles and must

be correctly accounted for. Advanced CFD modelling also needs

to be extended to more topics of interest in grate-fired boilers

burning biomass, e.g., formation of aerosol particles, which

covers at least sizes and compositions and formation of PCDD/

PCDF. Advanced CFD modelling also calls for more compre-

hensive experimentation for providing reliable data as inputs

or for validation.

Acknowledgements

The research on grate-firing of biomass was financially

supported by Grant PSO 4792, ‘‘Grate firing of biomass—Measure-

ments, validation and demonstration’’. The authors would like to

thank other project partners, Søren Lovmand Hvid, Torben Hille,

Torben Kvist Jensen, Ejvind Larsen, Marius Kildsig, and Bo Sander

(DONG Energy), Peter Glarborg, Peter Arendt Jensen, Haosheng

Zhou (DTU), Sønnik Clausen (Risø National Laboratory), and

Kenneth Jørgensen (BWV), for their helpful discussions during

all the project meetings. The authors are also grateful to seven

anonymous reviewers and Thomas Condra (AAU) for their

valuable comments that helped improve the quality of this paper.

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